To detect the current of an inverter, while suppressing the cost increase in a circuit system and, at the same time, maintaining the system in a small size by calculating the phase current values of other two phases, based on the processed results of a sample hold circuit, A/D conversion circuit, etc., which process the phase current value of one phase at prescribed timing.

Resistors 7C and 7V are respectively connected to U- and V- phases and detect the quantities of currents flowing to transistors 5U and 5V or feedback diodes 6U and 6V. The detected analog quantities of the currents are sent to a sample hold circuit 10b and further to a register circuit 10d, through an A/D converter 10c and stored in a memory 10e. Of the U-, V-, and W-phase currents, the U- and V-phase currents are alternately detected, and when the U-phase current is detected, the V-phase current is subjected to approximate calculation. When on the contrary the V-phase current is detected, the U-phase current is subjected to approximate calculation. A three-phase bridge circuit 2 finds the phase current of W-phase from a relation between the current values of the three phases such that the sum of the current values becomes zero.