diff --git a/src/renderer/README.md b/src/renderer/README.md
new file mode 100644
index 000000000..8ea46125b
--- /dev/null
+++ b/src/renderer/README.md
@@ -0,0 +1,15 @@
+# renderer/
+
+`renderer/` is the directory for the candidate window GUI process.
+
+## Overview
+
+The candidate window GUI process receives commands from the Mozc clients via
+IPC. The commands are defined in
+[`protocol/candidates.proto`](../protocol/candidates.proto).
+
+## Locations of main function per platform
+
+*   Linux (Qt): [`qt/qt_renderer_main.cc`](qt/qt_renderer_main.cc)
+*   macOS: [`mac/mac_renderer_main.cc`](mac/mac_renderer_main.cc)
+*   Windows: [`win32/win32_renderer_main.cc`](win32/win32_renderer_main.cc)