An image forming apparatus including an image forming unit, which is detachably mounted to the image forming apparatus. The image forming unit includes a storage unit configured to store a computer program for forming an image on the sheet; a detector configured to detect whether the image forming unit and the controller are connected to each other; a door to be opened to draw out the image forming unit from the image forming apparatus; and a locking unit configured to switch between a locked state and an unlocked state. The controller is configured to rewrite the computer program stored in the storage unit under a state in which the locking unit is in the locked state and the detector detects the connection between the image forming unit and the controller.