According to an embodiment, a power source circuit includes a switching element that is connected between an input terminal and an output terminal, a driving circuit that supplies a PWM driving signal to the switching element, a first control path that integrates a differential voltage between an output voltage and a reference voltage to output a first control signal, a second control path that converts the differential voltage into a digital signal to output a second control signal, and a PWM signal generation circuit that generates a PWM signal dependent on the first and second control signals.