Abstract: A flat tube includes pairs of dimple portions in a longitudinal direction thereof. Each of the pairs has a first dimple portion and a second dimple portion, which are slanted with respect to an intermediate line extending in the longitudinal direction at an intermediate position in a lateral direction so that inner end wall portions of the first and second dimple portions are located on the intermediate line, and so that the outer end wall potions thereof depart from the intermediate line in the lateral direction to be toward outer sides of the tube, respectively. The pairs of dimple portions are set to satisfy a relationship expressed by an inequality: 3?L2/L1?12, where L1 is a longitudinal projection length obtained by projecting one of the first and second dimple portions on the intermediate line, and L2 is a longitudinal pitch between adjacent pairs.