An elongated attachment hole is formed on a housing containing a magnetic sensor, a thin plate-like hole cover is placed on the housing to cover the elongated attachment hole, a single attachment bolt is disposed in such a manner as to go through the hole cover and the elongated attachment hole, and the attachment bolt is screwed in a screw hole of a cylinder body, thereby fixation of the housing on the cylinder body and fixation of the hole cover on the housing are performed at the same time by using the single attachment bolt.