diff --git a/src/shrpx-unittest.cc b/src/shrpx-unittest.cc
index d814296e..0de07c3d 100644
--- a/src/shrpx-unittest.cc
+++ b/src/shrpx-unittest.cc
@@ -120,6 +120,8 @@ int main(int argc, char* argv[])
                    shrpx::test_util_to_base64) ||
       !CU_add_test(pSuite, "util_percent_encode_token",
                    shrpx::test_util_percent_encode_token) ||
+      !CU_add_test(pSuite, "util_quote_string",
+                   shrpx::test_util_quote_string) ||
       !CU_add_test(pSuite, "util_utox", shrpx::test_util_utox) ||
       !CU_add_test(pSuite, "gzip_inflate", test_nghttp2_gzip_inflate)) {
      CU_cleanup_registry();
diff --git a/src/shrpx_https_upstream.cc b/src/shrpx_https_upstream.cc
index 3c55ceab..621c72a4 100644
--- a/src/shrpx_https_upstream.cc
+++ b/src/shrpx_https_upstream.cc
@@ -820,9 +820,11 @@ int HttpsUpstream::on_downstream_header_complete(Downstream *downstream)
 
       for(auto& altsvc : get_config()->altsvcs) {
         hdrs += util::percent_encode_token(altsvc.protocol_id);
-        hdrs += "=";
+        hdrs += "=\"";
+        hdrs += util::quote_string(std::string(altsvc.host, altsvc.host_len));
+        hdrs += ":";
         hdrs += util::utos(altsvc.port);
-        hdrs += ", ";
+        hdrs += "\", ";
       }
 
       hdrs[hdrs.size() - 2] = '\r';
diff --git a/src/util.cc b/src/util.cc
index 9fa93326..99b096df 100644
--- a/src/util.cc
+++ b/src/util.cc
@@ -141,6 +141,28 @@ std::string percentDecode
   return result;
 }
 
+std::string quote_string(const std::string& target)
+{
+  auto cnt = std::count(std::begin(target), std::end(target), '"');
+
+  if(cnt == 0) {
+    return target;
+  }
+
+  std::string res;
+  res.reserve(target.size() + cnt);
+
+  for(auto c : target) {
+    if(c == '"') {
+      res += "\\\"";
+    } else {
+      res += c;
+    }
+  }
+
+  return res;
+}
+
 std::string http_date(time_t t)
 {
   char buf[32];
diff --git a/src/util.h b/src/util.h
index 97fb13e3..f914a9cc 100644
--- a/src/util.h
+++ b/src/util.h
@@ -202,6 +202,10 @@ std::string percentDecode
 // Percent encode |target| if character is not in token or '%'.
 std::string percent_encode_token(const std::string& target);
 
+// Returns quotedString version of |target|.  Currently, this function
+// just replace '"' with '\"'.
+std::string quote_string(const std::string& target);
+
 std::string format_hex(const unsigned char *s, size_t len);
 
 std::string http_date(time_t t);
diff --git a/src/util_test.cc b/src/util_test.cc
index 3556f164..bc301399 100644
--- a/src/util_test.cc
+++ b/src/util_test.cc
@@ -101,6 +101,13 @@ void test_util_percent_encode_token(void)
   CU_ASSERT("http%202" == util::percent_encode_token("http 2"));
 }
 
+void test_util_quote_string(void)
+{
+  CU_ASSERT("alpha" == util::quote_string("alpha"));
+  CU_ASSERT("" == util::quote_string(""));
+  CU_ASSERT("\\\"alpha\\\"" == util::quote_string("\"alpha\""));
+}
+
 void test_util_utox(void)
 {
   CU_ASSERT("0" == util::utox(0));
diff --git a/src/util_test.h b/src/util_test.h
index 8a85ee89..d0220a88 100644
--- a/src/util_test.h
+++ b/src/util_test.h
@@ -32,6 +32,7 @@ void test_util_strieq(void);
 void test_util_inp_strlower(void);
 void test_util_to_base64(void);
 void test_util_percent_encode_token(void);
+void test_util_quote_string(void);
 void test_util_utox(void);
 
 } // namespace shrpx