A heat exchanging tube is provided with a flat tube main body having a predetermined length and a plurality of refrigerant passages extending in a tube longitudinal direction and arranged in a tube widthwise direction. The following relational equations (a) to (c) are satisfied: W=6 to 18 mm . . . (a); Ac/At.times.100=50 to 70% . . . (b) and P/L.times.100=350 to 450% . . . (c), where "W" is a width of the tube main body, "Ac" is a total cross-sectional area of the refrigerant passages, "At" is a total cross-sectional area of the tube main body ( including the refrigerant passages), "L" is an external perimeter of the tube main body and "P" is a total inner perimeter of the refrigerant passages. With this tube, enough pressure strength can be obtained and the passage resistance can be decreased while keeping the light weight, and further the heat exchanging performance can be improved.