Two systems consisting of an on-off control section corresponding to the saturation drive system and a constant voltage control section corresponding to the constant voltage drive system are provided as a circuit for controlling the conduction state of a transistor QP.sub.11 provided at the output section. The constant voltage unit comprises an operational amplifier A.sub.1, which monitors the output voltage Vout and feedback-controls the gate voltage of QP.sub.11. On the other hand, the on-off control section delivers a control signal for turning on and off QP.sub.11 corresponding to an input signal input to IN-U through inverters 40 and 42. Switches 46 and 48 for selectively applying the output from the control unit of either one of these two systems are provided to select the switch to be turned on based on a mode signal input to SW. This configuration enables the realization of different drive systems and restricts increase in circuit size while commonly using the output unit configured with comparatively large transistors. In addition, since the saturation drive system can be realized without depending on feedback control, high control responsiveness is secured to appropriately perform operation at high frequencies such as switching operations by PWM control.