Self-tapping screw

A screw comprising a screw shaft part, a drill part with a sharp end at an extreme end of the screw shaft part and a head part arranged at a rear end of said screw shaft part having an inverse truncated cone shape with an outer circumference of a curved concave surface and an upper surface with an engaging groove in which a screw driver tool is adapted to be installed, said upper surface of the head part including an edge part around said engaging groove formed with a spherical concave surface, said head part including a large diameter part extending from an outer circumferential edge of the upper surface to a lower end, and a small diameter part concentrically and integrally connected to the lower end of the large diameter part and extending to said screw shaft part, an outer circumference of said small diameter part constituting a second seat surface forming a part of the curved concave surface and having a large curvature at a terminal end near said large diameter part and a gentle curvature with a diameter gradually decreasing toward said screw shaft part, an outer circumference of said large diameter part constituting a first seat surface forming a part of the curved concave surface and having a diameter gradually decreasing toward the terminal end of said second seat surface with a curvature approximately equal to that near the terminal end of said second seat surface so as to form two-stage curved concave surfaces and an annular projecting edge at a connection part between said first seat surface and said second seat surface.