Express Routes - Issues with multiple files

I'm starting to learn Node.js with Express, but i've got few issues with the routing system.

In my app.js (my main file), i have :

var express = require('express');
var path = require('path');
var routes = require('./routes/index');
var admin = require('./routes/admin');
...
app.use('/', routes);
app.use('/admin, admin);

In the index.js :

var express = require('express');
var router = express.Router();

/* GET home page. */
router.get('/', function(req, res, next) {
  res.render('index', {
    title: 'Express'
  });
});

The root url work great, and it return correctly the jade template 'index'.

I'm trying to do the same thing, in a different file, for an 'admin' template (wich is in './routes/admin'). There is the point, when i go to the localhost:5000/admin (for exemple), it return a 404 error.

There is my admin.js file :

var express = require('express');
var router = express.Router();

router.get('/admin', function(req, res, next) {
  res.render('admin', {
    title: 'Connexion'
  });
});

Both of index.js and admin.js contain module.exports = router;

If someone got an idea or want to comment, you're welcome.

Thanks a lot (and forgive my english !)