PURPOSE: To prevent to lower a switching speed of a MOS power transistor(TR) by controlling the MOS TR in response to a voltage fed to its gate from an amplifier circuit.

CONSTITUTION: When a current flowing from an output 40a is increased due to any cause, a detection voltage of a detection circuit 50 is decreased. An amplifier circuit 10 compares a reference voltage with the detection voltage and decreases a gate voltage in this case to increase a source-drain current. Moreover, an error voltage detection circuit 60 compares the reference voltage with the detection voltage and gives a difference voltage to an absolute value detection circuit 70. The amplifier circuit 10 increases a gate current able to control a gate of a MOS transistor(TR) 40 based on a current fed from a signal generating circuit 80 thereby increasing the operating speed of the MOS TR 40.