To provide a cylinder which is excellent in operating characteristic while carrying out miniaturization.

Circumferential grooves 12, 13 are arranged on the outer peripheral side of a piston main body 5, and an annular packing 11 is mounted on the circumferential grooves 12, 13. The packing 11 is formed in a cross sectional nearly L-shape composing a base part 14 and an outer peripheral lip part 15 which extends from the base part 14. The groove widths of the circumferential grooves 12, 13 are formed in the width of such extent that the base part 14 is housed, and the base part 14 is formed in such constitution that an outer peripheral lip part 15 top end in a condition mounted on the circumferential grooves 12, 13 and a piston 5 main body end surface are positioned on the same position in relation to the thickness direction of the piston 5. The thickness of the piston 5 is sufficiently formed thinly comparing with the case that an U-packing is used, and thereby, it is possible to carry out downsizing of the piston 5 so as to carry out downsizing of a cylinder. Since the packing 11 is a lip packing after all, friction resistance between a cylinder tube inner circumferential surface 2a and the packing 11 is reduced, and also operating characteristic after pressure leaving is improved.