The present invention relates to a cylinder which has hollow portion therein; a piston rod which is inserted through an upper side of the cylinder and has a groove pattern having plurality of convex portions and concave portions which are alternately formed; a piston valve which is connected with a lower end of the piston rod so as to be reciprocated in the cylinder and has a fluid passing hole which is formed in an up and down direction; a rod guide which closes up an upper end of the cylinder tightly and guides an up-down movement of the piston rod and which has a through-hole at a side portion thereof; and a sensor module which is installed to the rod guide so as to detect a movement of the piston rod by using a variation in magnetic field generated when the piston rod is moved.