A bidirectional main switch includes two main switches M1 and M2 formed of two main MOSFET'S Q1 and Q2, respectively; a bidirectional mirror switch including two mirror switches M3 and M4 formed of two mirror MOSFET'S Q3 and Q4, respectively, both being formed so as to allow a small current (a mirror current) to flow therein in a specified ratio to a current in the main MOSFET'S Q1 and Q2; and an operational amplifier Op1 forming feed back amplifying circuit having power supplied from two power sources. Thus, a bidirectional current flowing in the bidirectional main switch for cutting off an excessive charge current and an excessive discharge current of a battery E can be detected by a simple circuit with low power losses and a high accuracy.