To provide a loosing action protecting screw impossible to be engaged with an engagement groove of a general screwdriver bit to prevent a removal of parts.

This loosing action protecting screw formed of a head parts 2 and a leg part 4 is formed with a guide hole 11 on the center line of the head part 2, and has the engagement groove 12 having a driving surface 13 to be engaged with a screwdriver bit 20 at positions divided into three in the radial direction around the guide hole 11, and this loosing action protecting screw can not be easily loosened by a general screwdriver. Parts fitted by using this screw can not be easily loosened for removal, a failure of the products due to an arbitrary repair and disassembling by a user can be prevented. The generation of unexpected accident by a user is reduced. Furthermore, the sufficient engagement with the screwdriver bit and the accurate transmission of the screw fastening torque can be obtained.