In a CO.sub.2 refrigerant cycle, a lubricating oil for a compressor has a compatibility relative to CO.sub.2 refrigerant, and the compatibility of the lubricating oil relative to the CO.sub.2 refrigerant at a pressure lower than a critical pressure of the CO.sub.2 refrigerant is lower than that at a pressure higher than the critical pressure of the CO.sub.2 refrigerant. Thus, in a low-pressure side such as an accumulator of the CO.sub.2 refrigerant cycle, because a liquid lubricating oil is separated with a liquid CO.sub.2 refrigerant, only the lubricating oil can be readily introduced into a suction side of the compressor, and it can prevent the liquid CO.sub.2 refrigerant from being sucked into the compressor. As a result, it can prevent a damage to the compressor while preventing deterioration of coefficient of performance of the CO.sub.2 refrigerant cycle.