Patent ID: 8369719

Claim:
An image forming apparatus, comprising: a fusing device including a heat source configured to produce heat, a fusing belt and a first temperature sensor, the fusing belt having a direct heated portion thereof that receives direct radiant-heat from the heat source, a support member supporting at least a portion of inner circumferential surface of the fusing belt, the support member having an opening through which heat produced by the heat source radiates to the direct heated portion of the fusing belt, the first temperature sensor being arranged to sense a temperature of the fusing belt at the direct heated portion of the fusing belt; and a control unit configured to calculate a temperature increase gradient VT of the fusing belt from a first temperature T 1 sensed by the first temperature sensor and a second temperature T 2 sensed by the first temperature sensor, the second temperature being sensed after a time delay A t from sensing of the first temperature and the temperature increase gradient being obtained according to an equation, VT=(T 2 −T 1 )/At, and to interrupt supply of power to the heat source based on a comparison of the calculated temperature increase gradient VT and a reference value Vr.