To provide a fixing device which is capable of adjusting a heating width of a fixing belt according to a size of a recording medium used, and of efficiently using heat for heating and fixing by preventing it from leaking to the outside as much as possible.

A fixing device comprises: an endless belt body; a pressure member which forms a nip part for a recording medium to pass by coming into contact with the outer periphery of the belt member; a pressing member which is disposed inside the belt body and presses the pressure member at the nip part via the belt body; and a heat generating body which is disposed inside the belt body and heats up the belt body. In addition, it includes a heat adjusting member which changes supply amount of radiation heat from the heat generating body to the belt body according to a size of a recording medium, in a position which is inside the belt body and is opposite to the pressing member via the heat generating member. The heat adjusting member and the pressing member are connected either directly or via a heat transfer member.