Method of manufacturing double pipe

A method of manufacturing a double pipe, the method comprising:

• forming a straight inner pipe having a predetermined length, the straight inner pipe defining a first and a second round groove;
• forming a straight outer pipe having a diameter larger than a diameter of the inner pipe, the straight outer pipe defining a first and a second aperture;
• inserting the inner pipe into the outer pipe such that the first groove of the inner pipe and the first aperture of the outer pipe are aligned;
• fixing a first end portion of the outer pipe to the inner pipe so as to form a first fixing portion;
• bending the outer pipe and the inner pipe at the same time at a predetermined portion so as to form at least one bending portion, after the forming of the first fixing portion;
• and
• fixing a second end portion of the outer pipe, opposite to the first end portion, to the inner pipe so as to form a second fixing portion, after the bending, the second groove of the inner pipe being aligned with the second aperture of the outer pipe when the fixing of the second end portion of the outer pipe is performed.