A heat exchanger unit includes first and second plate heat exchangers disposed in series along a refrigerant flow direction. A refrigerant flows from the first plate heat exchanger to the second plate heat exchanger when the heat exchanger unit operates as an evaporator to heat the refrigerant, and the refrigerant flows from the second plate heat exchanger to the first plate heat exchanger when the heat exchanger unit operates as a condenser to cool the refrigerant. The first and second plate heat exchangers have first and second gas-liquid mixing structures to promote gas-liquid mixing of the refrigerant when the heat exchanger unit heats the refrigerant. The first and second gas-liquid mixing structures are configured such that pressure loss becomes larger when the gas-liquid mixing action becomes higher and such that the gas-liquid mixing action of the first gas-liquid mixing structure is higher than the gas-liquid mixing action of the second gas-liquid mixing structure.