PURPOSE: To make the surface pressure at the time of locking low, and durability and pressure proof high by having a first pipe and a second pipe which is connected with the first pipe through a seal material, and an engagement protrusion, and by forming out of a coupler divided in a radius direction, and of a sleeve for restricting the extension of the coupler.

CONSTITUTION: A coupling A is formed out of a first pipe 1, a second pipe 2, a coupler 3 on the pipes 1, 2, and of a sleeve 4 fitted on the coupler 3. An engagement groove 5 is formed on the surface of the first pipe 1, and an O-ring 6 of a seal material is inserted into an inner circumferential surface. A bead part 10 for forming an engagement stop groove 9 in a circumferential direction, is provided on the second pipe 2. The coupler 3 has a shape of the pipe provided with an engagement protrusion 11 as well as an engagement stop protrusion 12 divided into two. The sleeve 4 is a pipe to be fitted into the coupler 3, and the extension of the coupler 3 is prevented thereby. Since the engagement part and the engagement protrusion as well as the engagement stop part and the engagement stop protrusion are in surface contact, durability and strength are increased.