diff --git a/src/h2load.cc b/src/h2load.cc
index 32cd5add..561e58c2 100644
--- a/src/h2load.cc
+++ b/src/h2load.cc
@@ -1589,7 +1589,7 @@ void Worker::free_client(Client *deleted_client) {
       client->req_todo = client->req_done;
       stats.req_todo += client->req_todo;
       auto index = &client - &clients[0];
-      clients[index] = NULL;
+      clients[index] = nullptr;
       return;
     }
   }
diff --git a/src/h2load_http3_session.cc b/src/h2load_http3_session.cc
index 573a8a14..6ca1e419 100644
--- a/src/h2load_http3_session.cc
+++ b/src/h2load_http3_session.cc
@@ -319,7 +319,8 @@ int Http3Session::init_conn() {
 
   int64_t ctrl_stream_id;
 
-  rv = ngtcp2_conn_open_uni_stream(client_->quic.conn, &ctrl_stream_id, NULL);
+  rv =
+      ngtcp2_conn_open_uni_stream(client_->quic.conn, &ctrl_stream_id, nullptr);
   if (rv != 0) {
     std::cerr << "ngtcp2_conn_open_uni_stream: " << ngtcp2_strerror(rv)
               << std::endl;
@@ -336,7 +337,7 @@ int Http3Session::init_conn() {
   int64_t qpack_enc_stream_id, qpack_dec_stream_id;
 
   rv = ngtcp2_conn_open_uni_stream(client_->quic.conn, &qpack_enc_stream_id,
-                                   NULL);
+                                   nullptr);
   if (rv != 0) {
     std::cerr << "ngtcp2_conn_open_uni_stream: " << ngtcp2_strerror(rv)
               << std::endl;
@@ -344,7 +345,7 @@ int Http3Session::init_conn() {
   }
 
   rv = ngtcp2_conn_open_uni_stream(client_->quic.conn, &qpack_dec_stream_id,
-                                   NULL);
+                                   nullptr);
   if (rv != 0) {
     std::cerr << "ngtcp2_conn_open_uni_stream: " << ngtcp2_strerror(rv)
               << std::endl;
diff --git a/src/inflatehd.cc b/src/inflatehd.cc
index 9a159ad6..f484042a 100644
--- a/src/inflatehd.cc
+++ b/src/inflatehd.cc
@@ -171,7 +171,7 @@ static int inflate_hd(json_t *obj, nghttp2_hd_inflater *inflater, int seq) {
 }
 
 static int perform(void) {
-  nghttp2_hd_inflater *inflater = NULL;
+  nghttp2_hd_inflater *inflater = nullptr;
   json_error_t error;
 
   auto json = json_loadf(stdin, 0, &error);
diff --git a/src/nghttpd.cc b/src/nghttpd.cc
index e2232403..b930d47a 100644
--- a/src/nghttpd.cc
+++ b/src/nghttpd.cc
@@ -55,7 +55,7 @@ namespace nghttp2 {
 namespace {
 int parse_push_config(Config &config, const char *optarg) {
   const char *eq = strchr(optarg, '=');
-  if (eq == NULL) {
+  if (eq == nullptr) {
     return -1;
   }
   auto &paths = config.push[std::string(optarg, eq)];
@@ -63,7 +63,7 @@ int parse_push_config(Config &config, const char *optarg) {
   auto i = eq + 1;
   for (;;) {
     const char *j = strchr(i, ',');
-    if (j == NULL) {
+    if (j == nullptr) {
       j = optarg_end;
     }
     paths.emplace_back(i, j);
diff --git a/src/shrpx-unittest.cc b/src/shrpx-unittest.cc
index 53b3ff03..24a1acdc 100644
--- a/src/shrpx-unittest.cc
+++ b/src/shrpx-unittest.cc
@@ -52,7 +52,7 @@ static int init_suite1(void) { return 0; }
 static int clean_suite1(void) { return 0; }
 
 int main(int argc, char *argv[]) {
-  CU_pSuite pSuite = NULL;
+  CU_pSuite pSuite = nullptr;
   unsigned int num_tests_failed;
 
   nghttp2::tls::libssl_init();
@@ -65,7 +65,7 @@ int main(int argc, char *argv[]) {
 
   // add a suite to the registry
   pSuite = CU_add_suite("shrpx_TestSuite", init_suite1, clean_suite1);
-  if (NULL == pSuite) {
+  if (nullptr == pSuite) {
     CU_cleanup_registry();
     return CU_get_error();
   }
diff --git a/src/shrpx_http2_session.cc b/src/shrpx_http2_session.cc
index 37a6eb40..4fcb90b7 100644
--- a/src/shrpx_http2_session.cc
+++ b/src/shrpx_http2_session.cc
@@ -1875,7 +1875,7 @@ void Http2Session::start_checking_connection() {
   SSLOG(INFO, this) << "Start checking connection";
   // If connection is down, we may get error when writing data.  Issue
   // ping frame to see whether connection is alive.
-  nghttp2_submit_ping(session_, NGHTTP2_FLAG_NONE, NULL);
+  nghttp2_submit_ping(session_, NGHTTP2_FLAG_NONE, nullptr);
 
   // set ping timeout and start timer again
   reset_connection_check_timer(CONNCHK_PING_TIMEOUT);
diff --git a/src/shrpx_log.cc b/src/shrpx_log.cc
index fe626d6b..de5e09fa 100644
--- a/src/shrpx_log.cc
+++ b/src/shrpx_log.cc
@@ -90,7 +90,7 @@ pthread_once_t lckey_once = PTHREAD_ONCE_INIT;
 } // namespace
 
 namespace {
-void make_key() { pthread_key_create(&lckey, NULL); }
+void make_key() { pthread_key_create(&lckey, nullptr); }
 } // namespace
 
 LogBuffer *get_logbuf() {
diff --git a/src/shrpx_log_config.cc b/src/shrpx_log_config.cc
index a23184c6..92eb0559 100644
--- a/src/shrpx_log_config.cc
+++ b/src/shrpx_log_config.cc
@@ -71,7 +71,7 @@ pthread_once_t lckey_once = PTHREAD_ONCE_INIT;
 } // namespace
 
 namespace {
-void make_key() { pthread_key_create(&lckey, NULL); }
+void make_key() { pthread_key_create(&lckey, nullptr); }
 } // namespace
 
 LogConfig *log_config() {
diff --git a/src/util.cc b/src/util.cc
index 7ee5acf9..2a2d211a 100644
--- a/src/util.cc
+++ b/src/util.cc
@@ -90,7 +90,7 @@ int nghttp2_inet_pton(int af, const char *src, void *dst) {
 
   int size = sizeof(struct in6_addr);
 
-  if (WSAStringToAddress(addr, af, NULL, (LPSOCKADDR)dst, &size) == 0)
+  if (WSAStringToAddress(addr, af, nullptr, (LPSOCKADDR)dst, &size) == 0)
     return 1;
   return 0;
 #  endif
diff --git a/src/util_test.cc b/src/util_test.cc
index 17d640d5..48d7efaf 100644
--- a/src/util_test.cc
+++ b/src/util_test.cc
@@ -207,7 +207,7 @@ void test_util_http_date(void) {
 }
 
 void test_util_select_h2(void) {
-  const unsigned char *out = NULL;
+  const unsigned char *out = nullptr;
   unsigned char outlen = 0;
 
   // Check single entry and select it.
@@ -217,7 +217,7 @@ void test_util_select_h2(void) {
       memcmp(NGHTTP2_PROTO_VERSION_ID, out, NGHTTP2_PROTO_VERSION_ID_LEN) == 0);
   CU_ASSERT(NGHTTP2_PROTO_VERSION_ID_LEN == outlen);
 
-  out = NULL;
+  out = nullptr;
   outlen = 0;
 
   // Check the case where id is correct but length is invalid and too
@@ -233,7 +233,7 @@ void test_util_select_h2(void) {
       memcmp(NGHTTP2_PROTO_VERSION_ID, out, NGHTTP2_PROTO_VERSION_ID_LEN) == 0);
   CU_ASSERT(NGHTTP2_PROTO_VERSION_ID_LEN == outlen);
 
-  out = NULL;
+  out = nullptr;
   outlen = 0;
 
   // Check the case that last entry's length is invalid and too long.