PURPOSE: To detect an overcurrent in small electric power consumption with high accuracy.

CONSTITUTION: A first transistor part 3 composed of a single NPN transistor 31 and four NPN transistors 41 to 44 having the same characteristic with the transistor 31, are respectively connected to both ends of an electric current detecting resistor 2 by putting an emitter on the resistor 2 side. The respective transistors 31, and 41 to 44 are connected by using a base in common with each other, and voltage is impressed between the base and the emitter from a voltage impressing part 5, and a collector is respectively connected to transistors 61 and 62 constituting a current mirror circuit. When band gap voltage ΔVBE being a voltage difference between the base and the emitter of transistor parts 3 and 4 is determined, since a current density ratio of both transistor parts 3 and 4 is determined, an overcurrent is detected by using this.