diff --git a/doc/nghttpx.h2r b/doc/nghttpx.h2r
index 74565e7e..7159e512 100644
--- a/doc/nghttpx.h2r
+++ b/doc/nghttpx.h2r
@@ -41,6 +41,33 @@ FILES
   :option:`--conf` option cannot be used in the configuration file and
   will be ignored if specified.
 
+Error log
+  Error log is written to stderr by default.  It can be configured
+  using :option:`--errorlog-file`.  The format of log message is as
+  follows:
+
+  <datetime> <master-pid> <current-pid> <thread-id> <level> (<filename>:<line>) <msg>
+
+  <datetime>
+    It is a conbination of date and time when the log is written.  It
+    is in ISO 8601 format.
+
+  <master-pid>
+    It is a master process ID.
+
+  <current-pid>
+    It is a process ID which writes this log.
+
+  <thread-id>
+    It is a thread ID which writes this log.  It would be unique
+    within <current-pid>.
+
+  <filename> and <line>
+    They are source file name, and line number which produce this log.
+
+  <msg>
+    It is a log message body.
+
 SIGNALS
 -------
 
diff --git a/src/shrpx_log.cc b/src/shrpx_log.cc
index 00f55d7e..330d539b 100644
--- a/src/shrpx_log.cc
+++ b/src/shrpx_log.cc
@@ -138,22 +138,14 @@ Log::~Log() {
   auto tty = lgconf->errorlog_tty;
 
   lgconf->update_tstamp(std::chrono::system_clock::now());
-  auto &time_local = lgconf->time_local;
 
-  if (severity_ == NOTICE) {
-    rv =
-        snprintf(buf, sizeof(buf), "%s PID%d [%s%s%s] %s\n", time_local.c_str(),
-                 config->pid, tty ? SEVERITY_COLOR[severity_] : "",
-                 SEVERITY_STR[severity_].c_str(), tty ? "\033[0m" : "",
-                 stream_.str().c_str());
-  } else {
-    rv = snprintf(buf, sizeof(buf), "%s PID%d [%s%s%s] %s%s:%d%s %s\n",
-                  time_local.c_str(), config->pid,
-                  tty ? SEVERITY_COLOR[severity_] : "",
-                  SEVERITY_STR[severity_].c_str(), tty ? "\033[0m" : "",
-                  tty ? "\033[1;30m" : "", filename_, linenum_,
-                  tty ? "\033[0m" : "", stream_.str().c_str());
-  }
+  // Error log format: <datetime> <master-pid> <current-pid>
+  // <thread-id> <level> (<filename>:<line>) <msg>
+  rv = snprintf(buf, sizeof(buf), "%s %d %d %s %s%s%s (%s:%d) %s\n",
+                lgconf->time_iso8601.c_str(), config->pid, lgconf->pid,
+                lgconf->thread_id.c_str(), tty ? SEVERITY_COLOR[severity_] : "",
+                SEVERITY_STR[severity_].c_str(), tty ? "\033[0m" : "",
+                filename_, linenum_, stream_.str().c_str());
 
   if (rv < 0) {
     return;
diff --git a/src/shrpx_log_config.cc b/src/shrpx_log_config.cc
index 57e97912..93735cce 100644
--- a/src/shrpx_log_config.cc
+++ b/src/shrpx_log_config.cc
@@ -23,6 +23,12 @@
  * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
 #include "shrpx_log_config.h"
+
+#include <unistd.h>
+
+#include <thread>
+#include <sstream>
+
 #include "util.h"
 
 using namespace nghttp2;
@@ -30,7 +36,14 @@ using namespace nghttp2;
 namespace shrpx {
 
 LogConfig::LogConfig()
-    : accesslog_fd(-1), errorlog_fd(-1), errorlog_tty(false) {}
+    : pid(getpid()), accesslog_fd(-1), errorlog_fd(-1), errorlog_tty(false) {
+  auto tid = std::this_thread::get_id();
+  auto tid_hash =
+      util::hash32(StringRef{reinterpret_cast<uint8_t *>(&tid),
+                             reinterpret_cast<uint8_t *>(&tid) + sizeof(tid)});
+  thread_id = util::format_hex(reinterpret_cast<uint8_t *>(&tid_hash),
+                               sizeof(tid_hash));
+}
 
 #ifndef NOTHREADS
 #ifdef HAVE_THREAD_LOCAL
diff --git a/src/shrpx_log_config.h b/src/shrpx_log_config.h
index 2f37f602..8fa44a9a 100644
--- a/src/shrpx_log_config.h
+++ b/src/shrpx_log_config.h
@@ -27,6 +27,8 @@
 
 #include "shrpx.h"
 
+#include <sys/types.h>
+
 #include <chrono>
 
 #include "template.h"
@@ -40,9 +42,11 @@ struct LogConfig {
   std::array<char, sizeof("03/Jul/2014:00:19:38 +0900")> time_local_buf;
   std::array<char, sizeof("2014-11-15T12:58:24.741+09:00")> time_iso8601_buf;
   std::array<char, sizeof("Mon, 10 Oct 2016 10:25:58 GMT")> time_http_buf;
+  std::string thread_id;
   StringRef time_local;
   StringRef time_iso8601;
   StringRef time_http;
+  pid_t pid;
   int accesslog_fd;
   int errorlog_fd;
   // true if errorlog_fd is referring to a terminal.