A fusing device of an image forming apparatus which applies principles of leverage, and an image forming apparatus containing such a device. The fusing device includes a fusing unit having a heating roller and a pressure roller facing each other and which rotate together, a cover which opens and closes the fusing unit, and a pressure and release unit, one end of which is rotatably fixed to a rotating shaft, the pressure and release unit containing a member having a pressuring unit which pressures the pressure roller towards the heating roller when the cover opens and shuts, and a pressured part, which is farther from the rotating shaft than the pressuring part and which receives pressure from the cover.