An image forming apparatus adapted for a smooth mounting/dismounting of a process unit and for the reduction of wasteful power consumption is provided.Unless a new image signal is applied after completion of an image forming operation, the apparatus starts a clocking operation by an internal timer (Step S101). If the apparatus is left standstill for a predetermined length of time (Step S102), a sleep process (Step S103) is executed to shift the apparatus to a power save mode to consume less power. When an outside cover is opened (Step S105), on the other hand, the apparatus is shifted to a replacement operation mode (Step S106) in which only the rotation of a development rotary is permitted. However, if the apparatus is left with the outside cover opened (Step S108), the sleep process is executed.