diff --git a/lib/node-loader-babel.js b/lib/node-loader-babel.js
index e41a59b..b268b39 100644
--- a/lib/node-loader-babel.js
+++ b/lib/node-loader-babel.js
@@ -19,7 +19,15 @@ function isBabelConfigFile(filename) {
 
 export async function load(url, context, defaultLoad) {
   if (useLoader(url)) {
-    const { source } = await defaultLoad(url, context, defaultLoad);
+    const { source, format } = await defaultLoad(url, context, defaultLoad);
+
+    // Skip transpilation of CommonJS modules.
+    // These modules are already preprocessed by Node.js,
+    // so we cannot parse the non-standard syntaxes like JSX and TypeScript.
+    // Their transpilation is better handled separately by @babel/register or @babel/node.
+    if (format !== "module") {
+      return { source, format };
+    }
 
     const filename = urlModule.fileURLToPath(url);
     // Babel config files can themselves be ES modules,
@@ -27,7 +35,7 @@ export async function load(url, context, defaultLoad) {
     if (isBabelConfigFile(filename)) {
       return {
         source,
-        format: /\.(c|m)?js$/.test(filename) ? "module" : "json",
+        format,
       };
     }
 
@@ -43,9 +51,9 @@ export async function load(url, context, defaultLoad) {
 
     return {
       source: transformed.code,
-      // Maybe a shaky assumption
-      // TODO: look at babel config to see whether it will output ESM/CJS or other formats
-      format: "module",
+      // NOTE: transform-modules-commonjs doesn't work properly.
+      //       We put a branch here just for consistency.
+      format: transformed.sourceType === "module" ? "module" : "commonjs",
     };
   } else {
     return defaultLoad(url, context, defaultLoad);
diff --git a/test/basic.test.js b/test/basic.test.js
index 930709a..6591008 100644
--- a/test/basic.test.js
+++ b/test/basic.test.js
@@ -10,6 +10,13 @@ describe(`basic babel usage`, () => {
     });
   });
 
+  it(`allows loading CJS modules`, async () => {
+    const example = await import("./fixtures/basic/cjs.cjs");
+    assert.deepEqual(example.default, {
+      cjs: "cjs",
+    });
+  });
+
   it(`supports ES module babel config files`, async () => {
     const mjsConfig = await import("./fixtures/mjs-config/main.js");
     assert.deepEqual(mjsConfig.default, {
diff --git a/test/fixtures/basic/cjs.cjs b/test/fixtures/basic/cjs.cjs
new file mode 100644
index 0000000..89d0db6
--- /dev/null
+++ b/test/fixtures/basic/cjs.cjs
@@ -0,0 +1 @@
+exports.cjs = "cjs";