Abstract: A heat exchanger includes a plurality of tubes and a plurality of projection portions. Each of the plurality of tubes has a flat cross section that is taken along a plane perpendicular to a tube longitudinal direction. The plurality of projection portions is formed on an inner wall of each of the plurality of tubes and is arranged in the tube longitudinal direction. Each projection portion projects from the inner wall in a tube short-side direction that extends along a transverse axis of the cross section. The projection portion has a curved surface part on a surface thereof. The projection portion has an elongated shape when observed in the tube short-side direction. The projection portion is angled relative to a tube long-side direction, which extends along a longitudinal axis of the cross section, in a manner similar to each other.