PURPOSE: To facilitate pattern design, by opening a gate for feeding operational power from the body section of one system to the control sections of respective systems on condition that the output from the body section of one system detected through a photo-coupler exceeds over a predetermined value.

CONSTITUTION: An IC power supply in a switching regulator(SR) 10 provides DC voltage Vcc to a control section 12 and a gate circuit 30. The control section 12 supplies a control voltage Vc to a body section 11. Light to be emitted from a photo diode 14a increases as the output voltage V1 from the body section 11 increases, thus reducing the resistance of a photo transistor 14b and increasing the feedback voltage Vf. The transistor 31 in the gate circuit 30 is conducted when the feedback voltage Vf exceeds over a predetermined value. Consequently, a transistor 32 is conducted and the source voltage Vcc is fed to the control section 22 of SR 20. The control section 22 supplies the control voltage Vc to the body section 21.