An image forming apparatus according to the present invention comprises three units, that is, an image forming unit 40, a fixing unit 60, and a transfer unit 50. The three units are mounted on a main frame 10 with a positional relationship upon being positioned therebetween. The image forming unit 40 is fixed to a predetermined position with respect to the main frame 10. Further, the main frame 10 comprises a positioning pin 14 arranged in a predetermined positional relationship with the image forming unit 40. The fixing unit 60 and the transfer unit 50 are positioned upon being engaged with the positioning pin 14. Consequently, there is no shift in position among the image forming unit 40, the transfer unit 50, and the fixing unit 60. The three units can be correctly mounted on the main frame 10.