To provide a thickness gauge capable of measuring a fine clearance below 150 m by a simple and sure method, and its manufacturing method.

A tapered surface is formed through a preparation process, wherein a base material 51 which is a tape-shaped thin plate is arranged beside a tool 50 which is parallel to the end face 50a of the tool 50 so that one surface of the base material 51 is brought into close contact with a reference curved surface, relative to a reference member (namely, the tool 50) on which the vertical reference curved surface is formed to the end face 50a which is an optional plane; and a processing process for forming a vertical processed curved surface Q2 to the end face, by processing the other surface of the base material 51 so as to have a curvature which is different from a curvature of the reference curved surface.