Patent ID: 8261641

Claim:
A screw comprising a recess for engaging with a driver bit, said recess being generally depression-like, said recess comprising a central portion and four blade engaging portions extending outwardly from said central portion, each of said blade engaging portions comprising a proximal end side portion extending outwardly in the radial direction of said screw and a distal end side portion extending further outwardly from said proximal end side portion, said proximal end side portions of said blade engaging portions being arranged at equal angular intervals, each of said blade engaging portions having a tightening side sidewall and a loosening side sidewall, said tightening side sidewall being a sidewall to exist in front of a blade of said driver bit when tightening said screw and said loosening side sidewall being a sidewall opposite to said tightening side sidewall to exist in front of said blade of said driver bit when loosening said screw, in each of said blade engaging portions, said tightening side sidewall in said distal end side portion bending at an oblique angle from said tightening side sidewall in said proximal end side portion away from said loosening side sidewall, said central portion having biting portions between said proximal end side portions of said four blade engaging portions, each of said biting portions having an inner wall protruding toward the axis of said screw and being inclined inwardly so that the deeper in said central portion, the narrower said central portion becomes.