diff --git a/toml/datetime.hpp b/toml/datetime.hpp
index 8c4cc3b..d3ae16d 100644
--- a/toml/datetime.hpp
+++ b/toml/datetime.hpp
@@ -1,7 +1,7 @@
 //     Copyright Toru Niina 2017.
 // Distributed under the MIT License.
-#ifndef TOML11_DATETIME
-#define TOML11_DATETIME
+#ifndef TOML11_DATETIME_HPP
+#define TOML11_DATETIME_HPP
 #include <chrono>
 #include <tuple>
 #include <array>
diff --git a/toml/exception.hpp b/toml/exception.hpp
index f92a7c1..1a3cd0c 100644
--- a/toml/exception.hpp
+++ b/toml/exception.hpp
@@ -1,7 +1,7 @@
 //     Copyright Toru Niina 2017.
 // Distributed under the MIT License.
-#ifndef TOML11_EXCEPTION
-#define TOML11_EXCEPTION
+#ifndef TOML11_EXCEPTION_HPP
+#define TOML11_EXCEPTION_HPP
 #include <stdexcept>
 #include <string>
 
diff --git a/toml/from_toml.hpp b/toml/from_toml.hpp
index a9348eb..f75b24b 100644
--- a/toml/from_toml.hpp
+++ b/toml/from_toml.hpp
@@ -1,7 +1,7 @@
 //     Copyright Toru Niina 2017.
 // Distributed under the MIT License.
-#ifndef TOML11_FROM_TOML
-#define TOML11_FROM_TOML
+#ifndef TOML11_FROM_TOML_HPP
+#define TOML11_FROM_TOML_HPP
 #include "get.hpp"
 
 namespace toml
diff --git a/toml/get.hpp b/toml/get.hpp
index 9d14e8c..ffc8747 100644
--- a/toml/get.hpp
+++ b/toml/get.hpp
@@ -1,7 +1,7 @@
 //     Copyright Toru Niina 2017.
 // Distributed under the MIT License.
-#ifndef TOML11_GET
-#define TOML11_GET
+#ifndef TOML11_GET_HPP
+#define TOML11_GET_HPP
 #include "from.hpp"
 #include "result.hpp"
 #include "value.hpp"
diff --git a/toml/region.hpp b/toml/region.hpp
index 0601bce..db830c7 100644
--- a/toml/region.hpp
+++ b/toml/region.hpp
@@ -1,7 +1,7 @@
 //     Copyright Toru Niina 2017.
 // Distributed under the MIT License.
-#ifndef TOML11_REGION_H
-#define TOML11_REGION_H
+#ifndef TOML11_REGION_HPP
+#define TOML11_REGION_HPP
 #include "exception.hpp"
 #include <memory>
 #include <vector>
diff --git a/toml/result.hpp b/toml/result.hpp
index 949b6f8..4930140 100644
--- a/toml/result.hpp
+++ b/toml/result.hpp
@@ -1,7 +1,7 @@
 //     Copyright Toru Niina 2017.
 // Distributed under the MIT License.
-#ifndef TOML11_RESULT_H
-#define TOML11_RESULT_H
+#ifndef TOML11_RESULT_HPP
+#define TOML11_RESULT_HPP
 #include "traits.hpp"
 #include <type_traits>
 #include <stdexcept>
diff --git a/toml/string.hpp b/toml/string.hpp
index eef932d..89e6d39 100644
--- a/toml/string.hpp
+++ b/toml/string.hpp
@@ -1,7 +1,7 @@
 //     Copyright Toru Niina 2017.
 // Distributed under the MIT License.
-#ifndef TOML11_STRING_H
-#define TOML11_STRING_H
+#ifndef TOML11_STRING_HPP
+#define TOML11_STRING_HPP
 #include <string>
 #include <cstdint>
 
diff --git a/toml/to_toml.hpp b/toml/to_toml.hpp
index eb7eff1..8fe2f69 100644
--- a/toml/to_toml.hpp
+++ b/toml/to_toml.hpp
@@ -1,7 +1,7 @@
 //     Copyright Toru Niina 2017.
 // Distributed under the MIT License.
-#ifndef TOML11_TO_TOML
-#define TOML11_TO_TOML
+#ifndef TOML11_TO_TOML_HPP
+#define TOML11_TO_TOML_HPP
 #include "value.hpp"
 
 namespace toml
diff --git a/toml/traits.hpp b/toml/traits.hpp
index 1991728..7f8f413 100644
--- a/toml/traits.hpp
+++ b/toml/traits.hpp
@@ -1,7 +1,7 @@
 //     Copyright Toru Niina 2017.
 // Distributed under the MIT License.
-#ifndef TOML11_TRAITS
-#define TOML11_TRAITS
+#ifndef TOML11_TRAITS_HPP
+#define TOML11_TRAITS_HPP
 #include <type_traits>
 #include <utility>
 #include <chrono>
diff --git a/toml/types.hpp b/toml/types.hpp
index 361ed10..db93320 100644
--- a/toml/types.hpp
+++ b/toml/types.hpp
@@ -1,7 +1,7 @@
 //     Copyright Toru Niina 2017.
 // Distributed under the MIT License.
-#ifndef TOML11_TYPES_H
-#define TOML11_TYPES_H
+#ifndef TOML11_TYPES_HPP
+#define TOML11_TYPES_HPP
 #include "datetime.hpp"
 #include "string.hpp"
 #include "traits.hpp"
diff --git a/toml/utility.hpp b/toml/utility.hpp
index a560f5d..01fe6d7 100644
--- a/toml/utility.hpp
+++ b/toml/utility.hpp
@@ -1,7 +1,7 @@
 //     Copyright Toru Niina 2017.
 // Distributed under the MIT License.
-#ifndef TOML11_UTILITY
-#define TOML11_UTILITY
+#ifndef TOML11_UTILITY_HPP
+#define TOML11_UTILITY_HPP
 #include "traits.hpp"
 #include <utility>
 #include <memory>
diff --git a/toml/value.hpp b/toml/value.hpp
index 00b790a..fa5670f 100644
--- a/toml/value.hpp
+++ b/toml/value.hpp
@@ -1,7 +1,7 @@
 //     Copyright Toru Niina 2017.
 // Distributed under the MIT License.
-#ifndef TOML11_VALUE
-#define TOML11_VALUE
+#ifndef TOML11_VALUE_HPP
+#define TOML11_VALUE_HPP
 #include "traits.hpp"
 #include "into.hpp"
 #include "utility.hpp"