PURPOSE: To reduce power loss by connecting a balance resistance in series with a transistor and arranging a current detector having one end connected to the joint of the transistor and the balance resistance and the other common end then setting the resistance of the current detection resistance to a sufficiently high value.

CONSTITUTION: Current detection output V0, resistances and currents have a relation shown by the formula. The current detection output V0 can be obtained as a total output on the emitter side of transistors Q1, Q2 connected with balance resistances R8, R9. Since current detection resistances R6, R7 do not enter into the current path of the transistor Q1, Q2 and approximately zero current flows through the resistances R6, R7, power loss can be reduced considerably and a small and inexpensive resistance can be employed.