A pair of cylinder holes 10 are formed in a cylinder tube 2 in the axial direction, a slit 25 is formed between these cylinder holes, and an iron plate 22 is inserted in the slit covering the whole range of the movement of the pistons 3 in the cylinder holes. Spacers 23 made of a synthetic resin are interposed on both sides of the iron plate 22 to reliably hold the iron plate 22 in the slit 25. The iron plate 22 disposed between the cylinder holes works to decrease the repulsive force acting between the inner magnets 14 of the pistons and produce an attracting force between the iron plate 22 and the inner magnets 14, and a contact surface pressure between the wear rings 9 of the pistons 3 and the wall surfaces of the cylinder holes 10 can be adjusted to a suitable value.