A reformer that directly receives heat and performs an ATR catalyst reaction and an SR catalyst reaction. The reformer includes: a reforming reactor to reform hydrogen containing fuel into reformed gas having abundant hydrogen by performing an ATR catalyst reaction and an SR catalyst reaction; a heat source contacting one side of the reforming reactor and providing the reforming reactor with heat; and an air feeder to feed the reforming reactor with air by an air flow control unit. Thus, the ATR catalyst reaction featuring a relatively short preheating time is performed while the reformer is initially operated, so that hydrogen can be produced when the reformer is initially operated, thereby efficiently operating a fuel cell.