Exemplary embodiments of the invention provide a sensor attachment mechanism capable of attaching a position detecting sensor having directivity in a proper direction to a fluid pressure cylinder having a rail extending along the length of a cylinder tube on the outer surface thereof. Embodiments include a position detecting sensor for sensing the operating position of a piston is attached with a sensor holder to a grooved rail extending along the length of a cylinder tube on the outer surface thereof. The position detecting sensor has directivity in magnetic detection. The sensor holder includes a base mounted on the rail and a sensor holding part extending from the base. The sensor holding part is close to or in contact with the surface of the cylinder tube outside rail walls. The sensor holding part has a holding guide for holding the position detecting sensor in a direction parallel to the axis line of the cylinder such that the directivity of the sensor in the magnetic detection is directed to substantially the center of the cylinder.