A piston position detecting device includes a position sensor, a fixing screw for attaching the position sensor to an outer periphery of the cylinder tube through the intermediary of a sensor attachment bracket, a nut plate into which the fixing screw is screwed, and a protecting cover covering the position sensor and the fixing screw, wherein the position sensor and the protecting cover are ableto be concurrently mounted to the cylinder tube using the fixing screw through the intermediary of a sensor attachment bracket by forming the nut plate from a portion of the protecting cover, and in addition, when the position sensor is position-adjusted, the position sensor and the protecting cover are enabled to be displaced together.