In an electric power steering device assisted by a brushless motor, an induced voltage from a brushless motor is detected when a main power supply is off. The motor is determined to be in a high rpm state when the induced voltage is in a first transient state from a voltage≤a high rpm detection threshold Th1 to a voltage≥Th1, thereby setting an intermittent excitation cycle of a revolution angle sensor connected to the motor to a short cycle. The motor is determined to be in a low rpm state when the induced voltage becomes a voltage≥a high rpm detection threshold Th2 after a wait period corresponding to an electrical angle between peaks or valleys of an output voltage has elapsed in a second transient state from a voltage≥Th1 to a voltage≤Th2, thereby setting the cycle to a long cycle.