PURPOSE: To improve the efficiency of a power source circuit by arranging a resistor on the connection route of one end of a current detection resistor with the base of a transistor and supplying an infinitesimal current to the resistor.

CONSTITUTION: When a load current IL is small, a certain voltage V1 is generated in the resistor 8 and the voltage of VS=RS×IL is generated in the current detection resistor 7. Thus, the voltage of V1+VS is generated between the base and emitter of an NPN transistor 5. In the section of VF>V1+VS with the voltage between the base and emitter defined as VF for turning ON the NPN transistor 5, the NPN transistor is not turned ON and a control signal 6 is not outputted. When VF=V1+VS is attained, the control signal 6 is outputted. Since a part of the voltage between the base and emitter of the NPN transistor 5 for detecting overcurrent and outputting the control signal is prevented from depending on the load current IL, the resistance value of the current detection resistor 7 can be made smaller than before and a generated voltage can be made small.