PURPOSE: To provide a cap for bolt hole shielding purpose which can be installed to a bolt hole smoothly and accurately in a simple operation, and can prevent the invasion of the rain water from the bolt hole securely.

CONSTITUTION: A cylinder 2 is provided at the lower side la of a head 1, the plane surfaces 2c are formed at the front side and the rear side surfaces of the cylinder 2, the cylinder 2 is divided into two circular arc walls 2a and 2b by a vertical groove 3 at the center of the plane surfaces 2c, thread ridges 5a and 5b are formed on the peripheral surfaces of the circular arc walls 2a and 2b leaving the front end diameter contraction parts 4a and 4b, a rotation operating member 6 is provided on the upper surface of the head 1, and a ring form projection 7 is provided to the outer side of the cylinder 2, so as to compose this cap for bolt hole shielding made of a plastic.