A medium registration apparatus for an image forming apparatus. The registration apparatus includes: a subassembly including a first guide plate that guides a medium along a medium transferring direction, and a side guide disposed adjacent to one side of the first guide plate, in parallel with the medium transferring direction and is a side registration reference; a subassembly moving guide that supports the subassembly, so that the subassembly can move in a direction crossing the medium transferring direction; and a subassembly driver that drives the subassembly. The image forming apparatus includes: the registration apparatus; an image forming part; a reverse roller that reverses a transferring direction of a medium printed on by the image forming part.