To solve a problem wherein a time is required for inserting a wrench in a narrow place, a place difficult to be reached by hand, or in case incapable of viewing an inside of a top part of a hexagon socket cap screw, although a hexagon wrench is easily to be inserted when the cap screw is positioned in a front face, and wherein the time is required for inserting the wrench, in particular, in case of an obstinate cap screw or in case of work repeated with tightening and loosening.

This hexagon socket head cap screw with a mark includes the mark matched to an internal six-divided hole on a side face of a screw head part.