This double pipe is formed by disposing an inner pipe (3) inside an outer pipe. The double pipe is used to perform heat exchange between a fluid flowing inside the inner pipe (3) and a fluid flowing between the inner pipe (3) and the outer pipe. In a straight pipe portion of the double pipe, the inner pipe (3) has a plurality of protruding portions (31) that are curved so as to protrude on the outer peripheral side and extend in the longitudinal direction. The protruding portions (31) are spirally displaced in the longitudinal direction of the double pipe. In the straight pipe portion, the inner peripheral surface of the outer pipe and the protruding portions (31) of the inner pipe (3) are in contact with each other, and an outer flow passage is formed which is defined at a plurality of circumferential places between the outer pipe and the inner pipe (3). In a cross-section of the straight pipe portion perpendicular to the longitudinal direction, when the maximum depth of each of the outer flow passages is D [mm], and the circumferential arc length is L [mm], the average D/L value of all the outer flow passages is greater than 0.09 and smaller than 0.20.