1. A switching power supply unit having a first switch and a second switch both connected in series to an input voltage source to supply power to a load from between said first switch and said second switch comprising: a current detection device to detect a current flowing through said first switch and to output as a current detection signal; a first current detection terminal to be connected to any one of one terminal and another terminal of said first switch; an overcurrent detection circuit to detect an overcurrent flowing through said first switch by comparing said current detection signal with an overcurrent detection threshold value; and an overcurrent detection threshold value selecting device to change said overcurrent detection threshold value of said overcurrent detection circuit depending on whether said first current detection terminal is connected to said one terminal of said first switch or said first current detection terminal is connected to said another terminal of said first switch.