To detect the elongation/contraction direction of a hydraulic buffer by an extremely simple structure without using any special space.

In a hydraulic buffer 6 elongated/contracted by the entering/ exiting of a piston rod 9 from a cylinder 7 according to the up-and-down movement of a car body, a case 15 having a detecting means 16 is fixed to the end part outer surface of the cylinder 7 close to the piston rod 9, a ring 18 is inserted into the piston rod 19 so as to be slid with soft tension while being held by the case 15 so as to be moved within a specified range, and the movement of the ring 18 made following the inserting/pulling-out of the piston rod 19 is detected by the detecting means of the case 15.