A fusing device includes a heating unit which provides heat for fixing toner on paper, a pressure unit which is aligned with the heating unit, and which provides pressure for fixing the toner onto the paper. The fusing device further includes a heat applying unit that heats the surface of the pressure unit so that the temperature difference between the surface of the pressure unit and the heating unit satisfies the regression equation .DELTA.T.ltoreq.-a*M+b, where .DELTA.T denotes the temperature difference, a and b denote empirically determined coefficients, and M denotes the molecular weight of the toner.