PURPOSE: To make it possible to reduce power consumption by dividing A driving pulse duration into one-phase exciting drive period, two-phase exciting drive period and one-phase exciting hold period, and controlling the supply power during the one-phase exciting hold period so that it will be lower than that during the exciting drive periods.

CONSTITUTION: Two-phase exciting driving pulse (TP) A and one-phase exciting driving pulse (OP) B corresponding to battery voltage, are calculated from characteristics curves by mapping, and the calculated values are stored. One- phase exciting holding pulse (HP) C is obtained by subtracting TP (A) and OP (B) from driving pulse duration, and the obtained value is stored. The output of TP (A), OP (B) and HP (C) is produced as counts on the timer built in ECU 33. The operation is performed by obtaining the timer count corresponding to TP (A), that corresponding to OP (B), and that corresponding to HP (C). The effective voltage of HP (C) is controlled so that it will be lower than those of TP (A) and OP (B).