To facilitate connecting the inner pipe of a double tube in which part of outer pipe and inner pipe are made common without deteriorating flowability fluid.

The double pipe 1 in which the inner pipe 12 is arranged in the outer pipe 11 and the outer pipe 11 and the inner pipe 12 are integrally formed by making part of them common is the connected. The hole 16 for connecting is bored in the common part 13 of the outer pipe 11 and the inner pipe 12. The hole 16 is incomplete bored holes having an uncut portion in the part. The cut piece 17 where the hole 16 is formed is obliquely projected into the inside of the inner pipe 12 so as to make an acute angle to the axial line of the hole 16.