A double pipe type heat exchanger, comprising: an inner pipe having a first flow path defined therein; and an outer pipe arranged around the inner pipe to define a second flow path between the inner pipe and the outer pipe, wherein: a longitudinally-extending gap exists between the inner pipe and the outer pipe; the inner pipe includes a spiral groove formed on an outer circumferential surface of the inner pipe to extend along a longitudinal direction of the inner pipe; the outer pipe includes a reduced diameter portion protruding inwardly so that the inner surface of the outer pipe is intermittently contacted with the outer circumferential surface of the inner pipe; the second flow path is defined by the spiral groove for allowing a fluid to flow spirally therethrough, and the gap for allowing the fluid to flow straightforward therethrough; the gap being configured to be at least intermittently blocked by the reduced diameter portion along the longitudinal direction of the inner and outer pipe (10, 20) to allow the fluid to flow through the spiral groove only; and the inner pipe includes a spiral ridge portion formed on the outer circumferential surface of the inner pipe to define the spiral groove, the reduced diameter portion being kept in contact with the spiral ridge portion. an inner pipe having a first flow path defined therein; and an outer pipe arranged around the inner pipe to define a second flow path between the inner pipe and the outer pipe, wherein: a longitudinally-extending gap exists between the inner pipe and the outer pipe; the inner pipe includes a spiral groove formed on an outer circumferential surface of the inner pipe to extend along a longitudinal direction of the inner pipe; the outer pipe includes a reduced diameter portion protruding inwardly so that the inner surface of the outer pipe is intermittently contacted with the outer circumferential surface of the inner pipe; the second flow path is defined by the spiral groove for allowing a fluid to flow spirally therethrough, and the gap for allowing the fluid to flow straightforward therethrough; the gap being configured to be at least intermittently blocked by the reduced diameter portion along the longitudinal direction of the inner and outer pipe (10, 20) to allow the fluid to flow through the spiral groove only; and the inner pipe includes a spiral ridge portion formed on the outer circumferential surface of the inner pipe to define the spiral groove, the reduced diameter portion being kept in contact with the spiral ridge portion. a longitudinally-extending gap exists between the inner pipe and the outer pipe; the inner pipe includes a spiral groove formed on an outer circumferential surface of the inner pipe to extend along a longitudinal direction of the inner pipe; the outer pipe includes a reduced diameter portion protruding inwardly so that the inner surface of the outer pipe is intermittently contacted with the outer circumferential surface of the inner pipe; the second flow path is defined by the spiral groove for allowing a fluid to flow spirally therethrough, and the gap for allowing the fluid to flow straightforward therethrough; the gap being configured to be at least intermittently blocked by the reduced diameter portion along the longitudinal direction of the inner and outer pipe (10, 20) to allow the fluid to flow through the spiral groove only; and the inner pipe includes a spiral ridge portion formed on the outer circumferential surface of the inner pipe to define the spiral groove, the reduced diameter portion being kept in contact with the spiral ridge portion.