An image forming device that uses a power transformer (T) such that no action delay and performance degradation/malfunction occur even in a power save mode, thereby achieving high energy efficiency. Circuits (2, 3, 4) of the image forming device are divided into to two blocks (A, B), and the power transformer (T) has two sets of windings (TA, TB), one for one of the blocks (A) that always needs power feeding and one for the other block (B) that needs power when necessary. Switches (SW) are provided on lines (L1 to L5) extending from the latter block (B) such that these switches are opened to interrupt power supply when the device is brought into the power save mode.