To prevent a screw from coming off, and prevent the screw from being loosened in removing the screw from a work.

A screw 10 consists of a head part 11 and a leg part 14 having a tapping screw part 15 which is integrated with the head part 11 and forms a female screw in a lower hole of a work at the tip. The tapping screw part 15 has 2-3 threads, and a thread-free part 17 of the diameter equal to or less than the diameter at the bottom of thread of the screw part 15 is formed continuous thereto, and the thread of the same pitch as that of the tapping screw part 15 is formed on a tightening part 18 between the screw-free part 17 and an under head part 13. When the screw is used for the work, no special working to prevent the screw from coming off from the work is not necessary, and when the screw is loosened, no screw comes off from the work and no screw is lost. Because the thread of the tightening part of the screw brings deformation to the female screw of the work, no screw is loosened even when the work is vibrated.