A fixing device and an image forming apparatus are provided that shorten warming-up time, reduce power consumption and deviation of a belt, and surely fix a toner image onto a sheet. Support members and a pressurizing member contact each other with pressure at an upstream side and a downstream side in a carrying direction of a recording medium. A first interposing portion of the belt disposed between the support members is inserted between the support members and the pressurizing member and is installed in a tensed state. A second interposing portion of the belt is installed in a non-tensed state, the pressurizing member contacts the outside of the first interposing portion to form the fixing nip portion. The support members contact the pressurizing member with different pressures at the upstream side and the downstream side in the carrying direction of the recording medium.