diff --git a/ext/objspace/objspace.c b/ext/objspace/objspace.c
index 0873ef9bcb..e410941c7d 100644
--- a/ext/objspace/objspace.c
+++ b/ext/objspace/objspace.c
@@ -12,9 +12,9 @@
 
 **********************************************************************/
 
+#include <ruby/io.h>
 #include "internal.h"
 #include <ruby/st.h>
-#include <ruby/io.h>
 #include <ruby/re.h>
 #include "node.h"
 #include "gc.h"
diff --git a/ext/objspace/objspace_dump.c b/ext/objspace/objspace_dump.c
index 19ba1f2e8b..8bd7c6769b 100644
--- a/ext/objspace/objspace_dump.c
+++ b/ext/objspace/objspace_dump.c
@@ -12,9 +12,9 @@
 
 **********************************************************************/
 
+#include "ruby/io.h"
 #include "internal.h"
 #include "ruby/debug.h"
-#include "ruby/io.h"
 #include "gc.h"
 #include "node.h"
 #include "vm_core.h"
diff --git a/ext/pty/pty.c b/ext/pty/pty.c
index 7726f154eb..fb9ff19a94 100644
--- a/ext/pty/pty.c
+++ b/ext/pty/pty.c
@@ -35,8 +35,8 @@
 #endif
 #include <ctype.h>
 
-#include "internal.h"
 #include "ruby/io.h"
+#include "internal.h"
 #include "ruby/util.h"
 
 #include <signal.h>
diff --git a/include/ruby/encoding.h b/include/ruby/encoding.h
index e6ceb19cdf..0c7e1e3d6a 100644
--- a/include/ruby/encoding.h
+++ b/include/ruby/encoding.h
@@ -12,6 +12,10 @@
 #ifndef RUBY_ENCODING_H
 #define RUBY_ENCODING_H 1
 
+#ifdef RUBY_INTERNAL_H
+#error "Include this file before internal.h"
+#endif
+
 #if defined(__cplusplus)
 extern "C" {
 #if 0
diff --git a/include/ruby/io.h b/include/ruby/io.h
index 60d6f6d32e..cde932ff32 100644
--- a/include/ruby/io.h
+++ b/include/ruby/io.h
@@ -12,6 +12,10 @@
 #ifndef RUBY_IO_H
 #define RUBY_IO_H 1
 
+#ifdef RUBY_INTERNAL_H
+#error "Include this file before internal.h"
+#endif
+
 #if defined(__cplusplus)
 extern "C" {
 #if 0
diff --git a/marshal.c b/marshal.c
index 1593ca2930..389005eb22 100644
--- a/marshal.c
+++ b/marshal.c
@@ -13,8 +13,8 @@
 # error too old GCC
 #endif
 
-#include "internal.h"
 #include "ruby/io.h"
+#include "internal.h"
 #include "ruby/st.h"
 #include "ruby/util.h"
 #include "encindex.h"
diff --git a/process.c b/process.c
index 79708b4d00..7bd9c1e5da 100644
--- a/process.c
+++ b/process.c
@@ -11,8 +11,9 @@
 
 **********************************************************************/
 
-#include "internal.h"
+#include "ruby/config.h"
 #include "ruby/io.h"
+#include "internal.h"
 #include "ruby/thread.h"
 #include "ruby/util.h"
 #include "vm_core.h"
diff --git a/thread.c b/thread.c
index c7d48c01db..3064c68e4f 100644
--- a/thread.c
+++ b/thread.c
@@ -63,10 +63,11 @@
 
 /* for model 2 */
 
+#include "ruby/config.h"
+#include "ruby/io.h"
 #include "eval_intern.h"
 #include "gc.h"
 #include "timev.h"
-#include "ruby/io.h"
 #include "ruby/thread.h"
 #include "ruby/thread_native.h"
 #include "ruby/debug.h"