A refrigeration system allows the refrigerant to circulate through a closed circulation channel. A dry evaporator is incorporated in the circulation channel. The dry evaporator is designed to keep a quality smaller than 1.0 in evaporating the refrigerant. The quantity of heat transfer per unit area, namely, a heat transfer coefficient depends on the quality. The heat transfer coefficient remarkably drops when the quality of the refrigerant exceeds a predetermined threshold level before the quality actually reaches 1.0. The quality of the refrigerant kept below the predetermined threshold level during vaporization of the refrigetant in the dry evaporator allows a reliable establishment of a higher performance of cooling. On the other hand, if a refrigerant completely evaporates in a dry evaporator in a conventional manner, the heat transfer coefficient of the refrigerant remarkably drops after the quality of the refrigerant exceeds the predetermined threshold level. Accordingly, the conventional dry evaporator is forced to absorb heat at a lower heat transfer coefficient, as compared with the present dry evaporator.