PURPOSE: To facilitate assembly by simplifying the structure of the main body case of a fixing device which fixes a toner image transferred to paper P.

CONSTITUTION: The lower case 35 is formed so as to integrate right and left support side plates 35a, by which the shaft-receiving parts of the right and left ends of a heat roller and pressure roller are supported, and an upstream guide plate 41 and downstream guide plate 42 arranged in the direction in which the pressure roller carries a recording medium. On the other hand, the right and left ends of a heater body inside the heat roller is covered by integrally providing downward side covers 36b on the right and left sides of the top plate 36a of the upper case 36 used for covering the upper part of the heat roller. Over the lower case 36 incorporating the heat roller and pressure roller, the upper case 36 is placed and fixed.