diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 73cc204..a7ec5ba 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -28,6 +28,9 @@ find_package(PkgConfig REQUIRED)
 
 pkg_check_modules(deps REQUIRED
     glib-2.0
+
+    xcb
+    xcb-aux
 )
 
 target_include_directories(${APP_NAME} SYSTEM
diff --git a/src/main.c b/src/main.c
index b48901a..18dbf8a 100644
--- a/src/main.c
+++ b/src/main.c
@@ -5,6 +5,9 @@
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+#include <xcb/xcb.h>
+#include <xcb/xcb_aux.h>
+#include <xcb/xproto.h>
 
 #include "backtrace.h"
 #include "buffer.h"
@@ -51,6 +54,34 @@ static void signal_child(int signum) {
   assert(res == 1);
 }
 
+static void check_other_wm(void) {
+  /* connect X server */
+  int default_screen;
+  xcb_connection_t *conn = xcb_connect(NULL, &default_screen);
+  int xcb_conn_error = xcb_connection_has_error(conn);
+  if (xcb_conn_error) fatal("cannot open display (error %d)", xcb_conn_error);
+
+  xcb_screen_t *screen = xcb_aux_get_screen(conn, default_screen);
+  xcb_window_t root = screen->root;
+
+  /* check other window manager running */
+  uint32_t mask = XCB_CW_EVENT_MASK;
+  const xcb_params_cw_t params = {
+    .event_mask = XCB_EVENT_MASK_SUBSTRUCTURE_REDIRECT,
+  };
+  xcb_void_cookie_t cookie =
+    xcb_aux_change_window_attributes_checked(conn, root, mask, &params);
+  if (xcb_request_check(conn, cookie)) {
+    fatal(
+      "another window manager is already running (cannot select "
+      "SubstructureRedirect)");
+  }
+
+  global_state.xcb_conn = conn;
+  global_state.default_screen = default_screen;
+  global_state.screen = screen;
+}
+
 int main(int argc, char *argv[]) {
   p_clear(&global_state, 1);
 
@@ -73,6 +104,8 @@ int main(int argc, char *argv[]) {
   sa.sa_flags = SA_NOCLDSTOP | SA_RESTART;
   sigaction(SIGCHLD, &sa, 0);
 
+  check_other_wm();
+
   if (global_state.loop == NULL) {
     global_state.loop = g_main_loop_new(NULL, FALSE);
     g_main_loop_run(global_state.loop);
diff --git a/src/types.h b/src/types.h
index f0d1975..00e6320 100644
--- a/src/types.h
+++ b/src/types.h
@@ -1,8 +1,16 @@
 #pragma once
 
 #include <glib.h>
+#include <xcb/xcb.h>
+#include <xcb/xproto.h>
 
 typedef struct global_state_t {
   bool need_restart;
   GMainLoop *loop;
+
+  xcb_connection_t *xcb_conn;
+  xcb_screen_t *screen;
+  int default_screen;
 } global_state_t;
+
+extern global_state_t global_state;