PURPOSE: To suppress a rush current by connecting a negative characteristic thermistor serious with a switching element which performs opening and closing of a power supply output.

CONSTITUTION: In a switching power supply circuit, a switching transistor(Tr) Q1 is connected with the primary winding of a transformer T and said TrQ1 is ON-OFF controlled by a switching controller 1. Also, rectifier circuits D1-D2 and smoothing circuits C1-C2 are connected with the secondary side of said transformer T. Then, a switching element Q2 composed of a negative characteristic thermostor RT and a power MOSFET is series-connected between said rectifier and smoothing circuits D2, C2 and a power output end V0. Further, a thermostat TH is connected between said rectifier and smoothing circuits D1, C1 and a TrQ2 gate. Thus, the service current to a load is limited, when the TrQ2 is once turned OFF at the abnormal time and thereafter is again turned ON as the abnormal condition is removed.