To miniaturize the entire runaway obstruction device by providing a reduction gear and two light shielding type photosensors in a servo motor.

A reduction gear 9 engaged with a gear 8 for a servo motor rotary shaft is provided and a CCW direction runaway obstruction light shielding type photosensor 5 and a CW direction runaway obstruction light shielding type photosensor 6 are attached to a position to detect runaway. When the servo motor 1 runs away and is rotated for more than a set rotation angle and a sensor light shielding plate 7 arrives at the installation position of the CW direction runaway obstruction light shielding type photosensor 6, CW direction operation inhibition input signals are supplied and inputted to the CW direction operation inhibition input terminal of a servo motor amplifier 2 and the servo motor 1 is thus stopped. When the servo motor 1 runs away and is rotated for more than the set rotation angle and the sensor light shielding plate 7 arrives at the installation position of the CCW direction runaway obstruction light shielding type photosensor 5, CCW direction operation inhibition input signals are supplied and inputted to the CCW direction operation inhibition input terminal of the servo motor amplifier 2 and the servo motor 1 is thus stopped.