To provide an encoder with a relatively simple structure, capable of being easily positioned and built with good repeatability and easily made more accurate than a conventional structure, and to provide a mounting method thereof.

In this encoder having an encoder body 1 to be mounted on a measured object 2, and according to this mounting method, an encoder mounting part of the measured object 2 has a spigot fit part 2a formed into a recessed shape while the encoder body 1 has, on its measured object mounting side, a spigot insertion part 1d that can be fitted to the measured object by being inserted into the fit part 2a. The insertion part 1d has diameter-expanding means 15 and 15a that can securely fix the insertion part 1d to the fit part 2a by expanding at least a part of the diameter thereof.