To prevent the adhesion of a concrete from making an obstacle for the screwing of a nut with the external threaded section of an anchor bolt, which is implanted in a concrete foundation, when the concrete is placed.

This protective cap is constituted by integrally forming a cap main body 2, which surrounds the external threaded section 31 of an anchor bolt 30, and a nut surrounding section 3, which is continued from the cap main body, and surrounds a nut being screwed to a specified location of the external threaded section 31. Then, the protective cap is fitted on the section 31 of the anchor bolt, which is fixed to an reinforcing iron being arranged in a concrete foundation 33, and is erected, prior to placement of the concrete. Then, the concrete placed in a molding die form, is prevented from adhering to the external threaded section 31 by covering the section 31.