To provide a bolt extremely effective in preventing theft of a number plate or the like of an automobile.

A umbrella like flange 2 is provided to the top of a screw shaft 1. A head 4 equipped with a turning operation part to which an ordinary turning tool is fitted is provided to the top of the umbrella like flange 2 through the medium of a constricted part 3 for rupturing whose diameter is made small. The constricted part 3 for rupturing is set to be cut at the prescribed turning torque. By doing so, when tightening the bolt to a prescribed turning torque with the ordinary turning tool, the constricted part 3 is ruptured to cut the head 4 of the bolt, whereby the umbrella like flange 2 with a smooth surface is left as the substantial bolt head.