A fixing device includes a cylindrical rotatable fixing belt, a flat heater, a safety device, a heater holding part and a pressuring member. The flat heater has a plurality of heat generating parts. The safety device is arranged across two adjacent heat generating parts to interrupt power supply to the heat generating parts in a case where temperature reaches a threshold value. The heater holding part holds the flat heater so that the flat heater comes into contact with an inner circumference face of the fixing belt. The pressuring member sandwiches the fixing belt with the flat heater to form a pressuring area for sandwiching and conveying a sheet between the fixing belt and the pressuring roller.