An inner pipe (2) is designed for a heat-transferring double pipe that exchanges heat between a fluid that flows through the interior of the inner pipe and a fluid that flows between the inner pipe and an outer pipe (10) that surrounds the inner pipe. The inner pipe has a first region (21) and a second region (22), which have transverse cross-sectional shapes that differ. The first region has a plurality of first protruding parts (211) that protrude outward and form a first recess-protrusion shape in which locations of the first protruding parts are offset helically in a longitudinal direction. The second region has a plurality of second protruding parts (221) that protrude outward and form a second recess-protrusion shape, in which locations of the second protruding parts are offset helically in the longitudinal direction. The number of second protruding parts is greater than the number of first protruding parts.