An electric power steering device to be assisted by a brushless motor , the electric power steering device comprising a cycle switching part configured to: detect an induced voltage from the brushless motor, which is revolved by an external force, via a rectification circuit when a main power supply is off; compare the induced voltage with a high rpm detection threshold (Th1) and a low rpm detection threshold (Th2); determine that the induced voltage is in a first transient state when the induced voltage transitions from a voltage equal to or less than the high rpm detection threshold to a voltage equal to or greater than the high rpm detection threshold; determine that the induced voltage is in a second transient state when the induced voltage transitions from a voltage equal to or greater than the high rpm detection threshold to a voltage equal to less than the high rpm detection threshold; determine that the induced voltage is in a third transient state when the induced voltage transitions from the first transient state to a voltage equal to or less than the low rpm detection threshold; determine that the brushless motor is in a high rpm state when the induced voltage is in the first transient state, thereby setting an intermittent excitation cycle of a revolution angle sensor connected to the brushless motor to a predetermined short cycle; and determine that the brushless motor is in a low rpm state when the induced voltage enters into the third transient state after a wait period corresponding to an electrical angle between peaks of an output voltage of the rectification circuit has elapsed in the second transient state, thereby setting the intermittent excitation cycle to a predetermined long cycle. detect an induced voltage from the brushless motor, which is revolved by an external force, via a rectification circuit when a main power supply is off; compare the induced voltage with a high rpm detection threshold (Th1) and a low rpm detection threshold (Th2); determine that the induced voltage is in a first transient state when the induced voltage transitions from a voltage equal to or less than the high rpm detection threshold to a voltage equal to or greater than the high rpm detection threshold; determine that the induced voltage is in a second transient state when the induced voltage transitions from a voltage equal to or greater than the high rpm detection threshold to a voltage equal to less than the high rpm detection threshold; determine that the induced voltage is in a third transient state when the induced voltage transitions from the first transient state to a voltage equal to or less than the low rpm detection threshold; determine that the brushless motor is in a high rpm state when the induced voltage is in the first transient state, thereby setting an intermittent excitation cycle of a revolution angle sensor connected to the brushless motor to a predetermined short cycle; and determine that the brushless motor is in a low rpm state when the induced voltage enters into the third transient state after a wait period corresponding to an electrical angle between peaks of an output voltage of the rectification circuit has elapsed in the second transient state, thereby setting the intermittent excitation cycle to a predetermined long cycle. <EOS>