A power supply circuit for measuring transient thermal resistances includes an inverter circuit provided on a primary side of a transformer and controlled by a PWM signal, a rectifier circuit provided on a secondary side of the transformer and including a DC reactor, and a control circuit controlling the PWM signal so as to output a pulsed output current from the rectifier circuit to a semiconductor device to be measured. The control circuit sets a first PWM frequency at rising timing of the output current, and sets a second PWM frequency when a predetermined time t1 elapses from the rising timing of the output current. The control circuit sets the first PWM frequency higher than the second PWM frequency.