In an electric power steering device to be assisted by a brushless motor, an induced voltage from a brushless motor revolved by an external force is detected via a rectification circuit when a main power supply is off, and this induced voltage is compared with a high rpm detection threshold and a return detection threshold for low rpm detection, which is lower than the high rpm detection threshold for determination. The brushless motor is determined to be in a high rpm state when the induced voltage is in a first transient state in which the induced voltage transitions from a voltage equal to or less than the high rpm detection threshold to a voltage equal to or more than the high rpm detection threshold, thereby setting an intermittent excitation cycle of a revolution angle sensor connected to the brushless motor to a short cycle determined in advance. The brushless motor is determined to be in a low rpm state when the induced voltage becomes a voltage equal to or less than the low rpm detection threshold after a wait period corresponding to an electrical angle between peaks or between valleys of an output voltage of a rectification circuit has elapsed in a second transient state in which the induced voltage transitions from a voltage equal to or more than the high rpm detection threshold to a voltage equal to less than the high rpm detection threshold, thereby setting the intermittent excitation cycle to a long cycle set in advance.