PURPOSE: To reduce the weight and strength of an inner pipe so as to manufacture it easily and economically by maintaining the inner and outer pipes movably in mutual axial engagement, and disengaging the pipes by means of an axial strong force.

CONSTITUTION: An inner pipe 14 is engaged with the inside of an outer pipe 12 with a coaxial interval thereto so as to form an axial passage that extends an overall length. Next, a recess 62 is circumferentially provided in the inner surface of the outer pipe 12 or the outer surface of the inner pipe 14 and the outer pipe 12 and the inner pipe 14 are maintained in a predetermined axial relationship by use of a spacer member 120 having a projection which fits into the recess. Th fit the projection into the recess 62, a strong axial force is needed, and separation of the two pipes 12, 14 also requires a similarly strong force. Thus, since the inner pipe 14 is supported by the outer pipe 12, the inner pipe 12 does not need to be increased in strength and weight and can be separated from the outer pipe 14 as necessary.