An object of the present invention is to provide a switching power supply which can stably determine whether an overcurrent occurs or not on the secondary side. A sample voltage corresponding to a load current value detected by detection resistors R7 and R8 is compared with a reference voltage by an input-side transistor Q4 and an output-side transistor Q3 which compose a first current mirror circuit 14, so that it is possible to determine whether an overcurrent occurs or not. Moreover, a current passing through the output circuit of the transistor Q3 is temperature-compensated by a second current mirror circuit 17 made up of transistors Q6 and Q5.