diff --git a/app/Main.hs b/app/Main.hs
index dd18056..19c43fe 100644
--- a/app/Main.hs
+++ b/app/Main.hs
@@ -6,7 +6,11 @@ module Main
 
 -- base
 import Control.Monad
-  ( void )
+  ( void, when )
+import Data.Maybe
+  ( isNothing )
+import System.Environment
+  ( lookupEnv, setEnv )
 import System.Exit
   ( ExitCode(..), exitSuccess, exitWith )
 import GHC.Conc
@@ -46,6 +50,12 @@ main = withCP65001 do
       = procs
   setNumCapabilities caps
 
+  -- Set GDK_SCALE = 2 by default,
+  -- unless GDK_SCALE is already set.
+  mbGdkScale <- lookupEnv "GDK_SCALE"
+  when (isNothing mbGdkScale) $
+    setEnv "GDK_SCALE" "2"
+
   ---------------------------------------------------------
   -- Run GTK application