PURPOSE: To simplify the structure of a device and to reduce malfunction caused by the influence of a magnetic field.

CONSTITUTION: A piston 5 is reciprocatably provided in a cylinder tube 2 composed of aluminum provided in a cylinder 1. A magnet 7 is attached to the piston 5. In addition, a pipe 13 which is made of aluminum and longitudinally extends is attached to the outer periphery of the cylinder tube 2. A movable body 14 which mutually attract the piston 5 and vice versa and is composed of a magnet is movably arrangedly provided in the pipe 13. Namely, the movable body 14 moves in the pipe 13 according to the movement of the piston 5. In addition, limit switches 15, 16 are severally attached to both the ends of the pipe 13, and prescrbie the stroke end of the piston 5 by detecting the movable body 14.