Fixing device and image forming apparatus

An image forming apparatus, comprising:

• an image forming unit for performing an operation of transferring a toner image to a transfer material, and
• a fixing unit for fixing the toner image by giving heat to the transfer material,
• the fixing unit including:
• a heat source for generating heat upon receiving the supply of power;
• a heating roller having the heat source mounted therein;
• a pressure roller disposed at a specified distance to the heating roller;
• a facing member disposed to face the heating roller;
• an endless fixing belt mounted on the facing member and the heating roller and pressed into contact with the pressure roller to form a nip portion;
• a temperature detecting member disposed to face the outer surface of the fixing belt for the detection of the surface temperature of the fixing belt;
• and
• an excessive temperature increase preventing member disposed to face a part of the outer circumferential surface of the heating roller where the fixing belt is not mounted for detecting the surface temperature of the heating roller and cutting off the power supply to the heat source when the surface temperature rises to or above a preset temperature.