An evaporator operated with the carbon dioxide gas, comprises at least a unit core including a plurality of heat transmission tubes having a path with a refrigerant flowing therein, a first tank connected to an end opening of the heat transmission tubes and formed with a refrigerant supply path and a second tank connected to the other end opening of the heat transmission tubes and formed with a refrigerant discharge path. The width L1 of the unit core is given as 50 mm.ltoreq.L1.ltoreq.175 mm. The equivalent diameter d of the refrigerant supply path of the first tank and the refrigerant discharge path of the second tank is given as 4.7 mm.ltoreq.d.ltoreq.9.6 mm.