An erroneous insertion preventing system includes a first cartridge including a first frame provided with a first engaging portion and a first discrimination portion, a second cartridge including a second frame provided with a second discrimination portion, and a main assembly of an image forming apparatus. The main assembly includes a movable member provided with a portion-to-be-engaged engageable with the first engaging portion and a regulating portion having a configuration corresponding to the first discrimination portion. The first cartridge is mounted in the main assembly without movement of the first discrimination portion being limited by the regulating portion. In a case that the second cartridge is attempted to be inserted into the main assembly, by insertion of the second cartridge into the main assembly, the regulating portion and the second discrimination portion interfere with each other to prevent mounting of the second cartridge in the main assembly.