PURPOSE: To steadily and securely perform assembling work by providing a recess at the middle of a groove which is cut at a screw head and to be fitted with a screw driver to prevent the screw driver from slipping in the direction of the groove.

CONSTITUTION: A recess 2 is provided at the middle of a groove 1 which is formed at a screw head 3 and to be fitted with a screw driver. The fitted connection between the groove 1 and the screw driver can therefore be steadied having no slippage of the screw driver. The shape of the recess 2 in the drawing way be an arc, a semisphere, a triangle or a trapezoid. Thus positional alignment becomes easy.