An image forming apparatus capable of reliably positioning a process unit in a main image forming apparatus body, without impairing ease of mounting/dismounting of the process unit achieved by a simple structure. The image forming apparatus including a convex section, provided on the process unit, that projects towards the insertion side; an engagement section, provided on the main apparatus body, that positions the process unit in a width direction by engaging with the convex section; and a guidance section, provided on at least one of a tip end of the convex section and the main apparatus body, that guides the convex section towards the guidance section when mounting the process unit.