A fixing device includes an endless-shaped fixing belt, a heater, a holding member, and a pressure member. The heater has a heat generating portion, and a seal portion that seals each end of the heat generating portion. The heater heats the fixing belt from inside the fixing belt with radiant heat. The holding member is disposed inside the fixing belt and slides against an inner circumferential surface of the fixing belt. The pressure member is pressed with a predetermined pressure against the holding member with the fixing belt therebetween, and thereby a fixing nip portion is formed between the pressure member and the fixing belt. An end-portion excessive temperature increase prevention member that blocks radiant heat is attached to each end portion of the heater. A first opening portion is formed in such part of the end-portion excessive temperature increase prevention member as faces the seal portion of the heater.