To contrive improvement in workability, easiness in assembly and reduction in the number of parts for a tightly sealing device by forming an annular seal groove on the outer periphery of a cap end outside of a stop ring or a screw, and fitting a seal between the groove and the internal periphery of a cylindrical body.

A piston rod is inserted in a cylinder tube 1 via a piston, a cap 4 is inserted therein from above of the cylinder tube 1, a thick wall part 15 is arranged below of the annular groove C and a snap ring 13 is fitted in the groove C. Then, when the piston rod is pulled upward, the upper end of the piston hits the lower end of a cap 4, and if the cap 4 is forcibly pulled upward, the cap 4 passes over the inside of the snap ring 13 to move outward, and the inside of the snap ring 13 is fitted between the outside contact surface 27b and the inside contact surface 27a, and the cap 4 is fixed. If an O ring 30 is pushed in downward from the upper side by means of a jig, etc., under this state, the O ring 30 is fitted in a seal groove 26. Thus the number of parts is small, and assembling property and workability are excellent.