There is provided a switching power supply unit having an overcurrent detection circuit capable of automatically selecting either a current detection method using an resistor or a current detection method using an external current detection resistor and of performing optimized overcurrent protection depending on applications. In the control circuit of the switching power supply unit for detecting a current flowing through a high-side switch as a current detection signal and comparing it with an overcurrent detection threshold value to detect an overcurrent and turning off the high-side switch to protect the switching power supply unit, a first current detection terminal connected to a power supply side terminal of the high-side switch or to a load side terminal of the high-side switch and a temperature compensating means to switch for temperature compensation on an overcurrent detection threshold value or on a current detection signal are provided.