A first fin is formed on a first inside wall surface to protrude therefrom. The first fin exchanges heat between fluid and the first inside wall surface. A first fluid passage is divided between a first region that is a space where the first fin is arranged, and a second region that is a space where the first fin is not arranged. Fluid flows from the second region into the first region. The first fin includes an inflow port through which fluid flows into the first region, and an outflow port through which fluid flows out of the first region. The first fin is formed to extend from the inflow port to the outflow port. An area of the inflow port is larger than a cross-sectional area of the first fluid passage when viewed in a flow direction of fluid.