To provide an overcurrent protective device which can activate an overcurrent protective function with average current, not with maximum current, in the case of a load whose required current amount increases only during starting as seen in induction load.

This power supply unit is formed so that current flowing through a switching element Q1 may be in proportion to the output current on a secondary side, therefore, when a value detected by a current detection resistor Rsp exceeds the maximum output current value 12 on the secondary side, a current detection circuit 11 on a primary side detects overcurrent, and operates so as to limit the output current by means of the PWM control. A current detection circuit 12 on the secondary side compares a current value detected by a resistor Rss with a current value I1 specified as an average current amount. When the detected current value exceeds the current value I1, a pulse width detection circuit 13 is operated, and it is discriminated that a defection signal from the current detection circuit 12 is inputted continuously for a prescribed time width or more, and the output current is limited by means of the PWM control.