To provide a screw capable of remarkably reducing coming out of the screw if an exclusive screw driver bit is used, and also allowing use of a screw driver bit with a standard cross slot and reducing coming out of the screw in some extent, and to provide the exclusive screw driver bit.

Out of a rotary part 13, a center part 17 and a base end side portion 18a of a blade engagement part 18 are configured the same as a standard cross slot. A fastening-side side wall face 20b or a loosening-side side wall face 21b of a tip end side portion 18b of the blade engagement part 18 is diagonally bent. The exclusive screw driver bit has a cross sectional shape corresponding to the rotary part 13. Coming out of the screw seldom occurs, as the exclusive screw driver bit seldom tilts with respect to the screw 1 and engagement between a blade part of the screw driver bit and the blade engagement part 18 is improved. A standard screw driver bit can also be used as the center part 17 and the base end side portion 18a of the blade engagement part 18 are the same as the standard cross slot.