A fixing device 5 includes a belt drive assist mechanism 70 configured to rotationally drive a fixing belt 26. The belt drive assist mechanism 70 is in contact with an inner surface of the fixing belt 26 and an outer diameter of an outer circumferential surface 71a is smaller than an inner diameter of the fixing belt 26 when the fixing belt 26 is in a circular shape. In addition, the belt drive assist mechanism 70 includes a cylindrical rotation member 71 and a pressing roller 72. The cylindrical rotation member 71 is rotationally driven by a motor 79. The pressing roller 72 is in pressure contact with the cylindrical rotation member 71 while interposing the fixing belt 26 therebetween and rotates following rotation of the fixing belt 26 rotationally driven by the cylindrical rotation member 71 or the pressure applying roller 19.