A position detecting sensor is mounted in a sensor attachment groove that opens on a side surface of an actuator and extends in an axial direction of the actuator. The position detecting sensor includes a housing that houses a sensor main body, and a guide member inserted into the sensor attachment groove. The guide member is of a shape such that, when the guide member is inserted into the sensor attachment groove, the guide member is required to be rotated in one direction around an axis parallel with the axis of the actuator and then rotated in a direction opposite to the one direction. After the guide member has been inserted into the sensor attachment groove, attachment thereof to the sensor attachment groove is completed by application of a fastener.