A position adjustment device is provided with an eccentric pin and a push rod. The eccentric pin has a rotating shaft part, and an eccentric shaft part inserted into an end in a circumferential direction of a lower-half inside member. The push rod is inserted into the lower-half inside member from an end surface in the circumferential direction of the lower-half inside member, the push rod being able to come into contact with the eccentric shaft part within the lower-half inside member. The rotating shaft part has a columnar shape with a rotational axis line. The eccentric shaft part is formed with: a side peripheral surface that comes into contact with the inner peripheral surface of an eccentric shaft hole in the lower-half inside member into which the eccentric shaft part is inserted, and a rod contact surface that comes into contact with the push rod.