A hydrogen producing device is mounted at an exhaust gas port of a vehicle to receive exhaust gas and waste heat as a heat source necessary for a reforming reaction with a catalyst member in a reaction chamber. The hydrogen producing device includes a heating chamber in which the reaction chamber is received, a fuel introducing tube disposed to introduce fuel to the reaction chamber, an air introducing tube disposed in the heating chamber to exchange heat with a reaction air thereinto and introducing the reaction air into the reaction chamber for the reforming reaction, and a product discharging tube disposed to discharge a hydrogen-rich synthesis gas generated in the reaction chamber.