PURPOSE: To eliminate the portion of a stroke sensor appearing outside an attenuator and simplify and miniaturize its structure by storing the bar section of the stroke sensor in the pipe-shaped piston rod of the tubular attenuator, and holding a circular section faced to the outer periphery of the bar section on caps of the cylinder side.

CONSTITUTION: The piston rod 32 of a tubular attenuator 24 expanded or shrunk through caps 36, 38 of a cylinder 28 is formed into a pipe shape. The bar section 50 of a stroke sensor is stored in the piston rod 32, a circular section 56 faced to the outer periphery of the bar section 50 is held on the caps 36, 38, and the stroke position of the tubular attenuator 24 is detected. The mechanical structure of the stroke sensor is simplified and miniaturized, the bar section 50 and the circular section 56 are protected from rain water and dust, and the duration can be improved.