A heat exchanger comprising: refrigerant causing disproportionation; a main heat exchange unit including a plurality of first heat transfer pipes arranged side by side; a sub-heat exchange unit including a plurality of second heat transfer pipes arranged side by side; and a relay unit including a plurality of relay passages connecting the plurality of first heat transfer pipes and the plurality of second heat transfer pipes, each of the plurality of relay passages having one inlet connected to a corresponding one of the plurality of second heat transfer pipes, and a plurality of outlets each connected to a corresponding one of the plurality of first heat transfer pipes, each of the plurality of relay passages distributing the refrigerant flowing from the one inlet, without merging streams of the refrigerant together, and causing the refrigerant to flow out of the plurality of outlets, characterized in that the sub-heat exchange unit is located below the main heat exchange unit, and in that the relay unit is configured to cause a smaller pressure loss of the refrigerant passing through the relay unit than a pressure loss of the refrigerant passing through the sub-heat exchange unit, wherein the relay unit is configured to cause a larger pressure loss of the refrigerant passing through the relay unit than a pressure loss of the refrigerant passing through the main heat exchange unit, and wherein each of the plurality of relay passages has a passage cross-sectional area equal to or more than a passage cross-sectional area of the corresponding one of the plurality of second heat transfer pipes connected to the one inlet, and equal to or less than a total of passage cross-sectional areas of the plurality of first heat transfer pipes connected to the plurality of outlets. refrigerant causing disproportionation; a main heat exchange unit including a plurality of first heat transfer pipes arranged side by side; a sub-heat exchange unit including a plurality of second heat transfer pipes arranged side by side; and a relay unit including a plurality of relay passages connecting the plurality of first heat transfer pipes and the plurality of second heat transfer pipes, each of the plurality of relay passages having one inlet connected to a corresponding one of the plurality of second heat transfer pipes, and a plurality of outlets each connected to a corresponding one of the plurality of first heat transfer pipes, each of the plurality of relay passages distributing the refrigerant flowing from the one inlet, without merging streams of the refrigerant together, and causing the refrigerant to flow out of the plurality of outlets, characterized in that the sub-heat exchange unit is located below the main heat exchange unit, and in that the relay unit is configured to cause a smaller pressure loss of the refrigerant passing through the relay unit than a pressure loss of the refrigerant passing through the sub-heat exchange unit, wherein the relay unit is configured to cause a larger pressure loss of the refrigerant passing through the relay unit than a pressure loss of the refrigerant passing through the main heat exchange unit, and wherein each of the plurality of relay passages has a passage cross-sectional area equal to or more than a passage cross-sectional area of the corresponding one of the plurality of second heat transfer pipes connected to the one inlet, and equal to or less than a total of passage cross-sectional areas of the plurality of first heat transfer pipes connected to the plurality of outlets.