To attain lightening of weight and improvement of durability, by providing a bag-shaped socket inserting an end part of a second pipe in an end part of a first pipe, also providing a spring, engaged with an engaging part formed in an end part of the second pipe to regulate its movement in a slip off direction, in this socket.

A socket 3 is fixedly caulked in an end part of a pipe 2, a plate spring 4 is provided in this socket 3. A swollen part 6 protruded in a diametric direction of the pipe 2 and a groove 8 fitting an O ring 7 are formed in the end part of the pipe 2. On the other hand, an end part of a pipe 9 is formed in a diametrically spread part 10, and a flange 11 is provided in the end of this diametrically spread part 10. When the end part of the pipe 2 is inserted in the end part of the pipe 9 in the case of connecting the pipes 2, 9, the flange 11, while press opening the plate spring 4, is inserted in the socket 3, and the flange 11 is engaged with a pawl 12 of the plate spring 4, to regulate movement of the pipe 9 in a slip off direction. Since a strong joint structure can be thus formed, workability can be markedly improved.