It is an object of the present invention to prevent recurrence of an overload condition after occurrence of the overload condition, thereby improving operability and safety in a drilling device. A motor 12 for rotating a drill is connected to an AC power source 10 through a motor control unit 28, a current detector 30, and a power switch 36. A magnet 16 is also connected to the AC power source 10 through the power switch 36 and a full-wave rectifier 18. The motor control unit 28 rotationally drives the motor on the basis of a signal sent from a main control unit according to a state in which a motor start switch 38 is on. The main control unit 14 controls the motor control unit 28 to gradually reduce a supply voltage to the motor 12 when the motor 12 becomes overloaded, to gradually increase the voltage to the normal power supply condition when the overload condition is vanished, and to stop power supply to the motor 12 if the overload condition continues for a predetermined period.