An operation switch sends an ON signal to a control processor indicating the operation switch is triggered by a user. A magnetic sensor sends a sensor signal to the control processor based on a displacement of a chip rod in a rearward direction. In the case where the sensor signal has not yet been sent, the control processor performs a control in a first mode where a brushless DC motor is not driven. In the case where sensor signal is sent, control processor performs a control in a second mode wherein the brushless DC motor is driven. An output in the first mode is lower than in the second mode. Because of this manner of construction, even if an ON input is triggered and held by the user, electric power is conserved while the operation switch is switched on and a hexagonal nut is not fastened, improving power consumption efficiency.