A step-like operation command signal is inputted into a time-constant circuit to generate a gradually rising operation command input voltage, which is supplied to a power supply unit. Enablement and disablement of the power supply unit is controlled on the basis of the operational command input voltage. When the operation command input voltage exceeds a first predetermined voltage level, a detection signal and a reference voltage are generated, and at the same time an error amplification circuit is enabled. On the other hand, when the operation command input voltage exceeds a second predetermined voltage level, a level shift voltage for initiating a soft start is generated, which enables execution of a soft start of the power supply unit without increasing its consumption current and reduction of the number of its terminals.