A position detecting sensor is mounted in a sensor attachment groove of a cylinder device. The sensor attachment groove includes a wide portion on the side of a groove bottom surface, and a narrow portion on the side of a side surface of the cylinder device. The position detecting sensor includes a mounting member having a width dimension larger than a groove width of the narrow portion. The mounting member includes a first engagement part and a second engagement part, which are flexible and are arranged in confronting relation, opening with a gap mutually therebetween in a widthwise direction of the sensor attachment groove.