A position-detecting sensor to which a mounting mechanism for a position-detecting sensor is applied is equipped with a holder capable of accommodating a sensor element in which a magnetic sensor or the like is disposed, and includes a flange on a lower end of the holder, which is inserted through a sensor mounting groove of a cylinder apparatus. The flange is formed in a plate shape, having a narrow portion and a widened portion that differ in width dimension. After insertion thereof through the sensor mounting groove such that side surfaces of the narrow portion are made parallel with the sensor mounting groove, the holder is turned, whereby the widened portion engages within and is retained respectively by a pair of recess portions in the sensor mounting groove.