An image forming apparatus that is capable of preventing a damage to a fusing belt or hazardous conditions due to the overheating of the fusing belt and a fusing device thereof are disclosed. The image forming apparatus has a fusing device including a fusing belt, a heat source disposed in the fusing belt, a support member to support the fusing belt, the support member having an opening through which heat emitted from the heat source passes to at least a portion of the fusing belt, and a first temperature sensor positioned to measure temperature of the fusing belt at the portion directly heated by radiant heat transmitted through the opening. A control unit of the image forming apparatus controls the heat source based on the temperature measured by the first temperature sensor.