A heat exchanger is used in a vapor-compression type refrigerator where a pressure of a refrigerant at a high-pressure portion reaches and exceeds a critical pressure. A low-pressure refrigerant flows through the heat exchanger. The heat exchanger comprises a flat tube; refrigerant channels included in the tube; and inner pillars disposed between the refrigerant channels. A tensile strength of material of the tube is defined as S [N/mm2]; of one of the refrigerant channels, a dimension approximately parallel with a major-axis direction of the tube, as Wp [mm]; and, of one of the pillars, a thickness approximately parallel with the major-axis direction of the tube, as Ti [mm]. Here, [447×Wp/{10^(1.54×log10S)}−533/{10^(1.98×log10S)}]≦Ti≦[447×Wp/{10^(1.54×log10S)}−533/{10^(1.98×log10S)}]×2.3.