To provide a fastening part protection device capable of improving a security function further.

This fastening part protection device is cmposed of a holder screw-fitted into a fastened object, a skirt surrounding the holder, a ball guide connected integrally with the skirt and forming a ball relief channel, a ball fitted into a ball holding channel and the ball relief channel of the skirt and engaging and disengaging with/from a ball locking channel of the holder, a stopper fitted into the skirt so as to prevent rotation and move in the direction of the axis of the ball guide and forming a cam slope in a base part, a spring for energizing the stopper to press the ball on the cam slope, and a dial lock provided in the skirt and unlocked when a mark on an outer peripheral face of a dial is aligned on a row of unlocking marks set in advance. When rotating the dial and setting it to the row of unlocking marks, the stopper can move, and the ball escapes from the ball locking channel to pull the skirt out of the holder.