An image forming device is disclosed which is provided with a simple lock mechanism for locking the position of the developing units on a rotary frame during toner supply. In the lock mechanism, a solenoid is turned off and a rod is moved downward by a spring so that a second member is rotated counterclockwise and a claw at the end thereof is engaged with a protruding portion of a first member. In this state, a rotary frame driving gear is prohibited from rotating, i.e., the rotary frame is locked. Thus, it is unnecessary to prepare a conventional two-way lock mechanism, and displacement of the rotary frame during toner supply is restrained with a simple structure.