A heat pump apparatus comprising: a heat exchanger including a plurality of thin-plate members spaced apart from one another, fluid flowing through the space, a flow channel formed in each of the plurality of thin-plate members, a medium flowing through the flow channel, the medium exchanging heat with the fluid, and a pair of headers (2, 3) each connecting ends of a corresponding one of two sides of the plurality of thin-plate members; and a refrigerant circuit connecting a compressor, a condenser, an expansion unit, and an evaporator by pipes and circulating refrigerant, wherein the evaporator is the heat exchanger, wherein the heat exchanger is arranged and connected so that the refrigerant flows into one of the pair of headers (2, 3) provided on a lower side in a direction of gravity, flows through the flow channel formed in each of the plurality of thin-plate members in a direction from the lower side toward an upper side in the direction of gravity, flows into an other one of the pair of headers (2, 3) provided on the upper side in the direction of gravity, and is discharged from the other one of the pair of headers (2, 3) on the upper side in the direction of gravity, wherein the heat exchanger is one of heat exchangers arranged in parallel on the upper side and on the lower side in the direction of gravity, wherein the heat exchangers arranged in parallel are connected in parallel, characterized in that one of the pair of headers (2, 3) provided on the lower side in the direction of gravity includes an outer pipe connecting the ends of one side of the plurality of thin-plate members; and an inner pipe provided inside the outer pipe, wherein the inner pipe has an inlet and a plurality of outlets, the refrigerant flowing into the inlet and being discharged from the plurality of outlets to the outer pipe, wherein the outer pipe has two closed ends, and a tube that forms a refrigerant inlet extends through a side wall of the outer pipe, and wherein the plurality of outlets are provided only in a lower part of the inner pipe in the direction of gravity. a heat exchanger including a plurality of thin-plate members spaced apart from one another, fluid flowing through the space, a flow channel formed in each of the plurality of thin-plate members, a medium flowing through the flow channel, the medium exchanging heat with the fluid, and a pair of headers (2, 3) each connecting ends of a corresponding one of two sides of the plurality of thin-plate members; and a plurality of thin-plate members spaced apart from one another, fluid flowing through the space, a flow channel formed in each of the plurality of thin-plate members, a medium flowing through the flow channel, the medium exchanging heat with the fluid, and a pair of headers (2, 3) each connecting ends of a corresponding one of two sides of the plurality of thin-plate members; and a refrigerant circuit connecting a compressor, a condenser, an expansion unit, and an evaporator by pipes and circulating refrigerant, wherein the evaporator is the heat exchanger, wherein the heat exchanger is arranged and connected so that the refrigerant flows into one of the pair of headers (2, 3) provided on a lower side in a direction of gravity, flows through the flow channel formed in each of the plurality of thin-plate members in a direction from the lower side toward an upper side in the direction of gravity, flows into an other one of the pair of headers (2, 3) provided on the upper side in the direction of gravity, and is discharged from the other one of the pair of headers (2, 3) on the upper side in the direction of gravity, wherein the heat exchanger is one of heat exchangers arranged in parallel on the upper side and on the lower side in the direction of gravity, wherein the heat exchangers arranged in parallel are connected in parallel, characterized in that one of the pair of headers (2, 3) provided on the lower side in the direction of gravity includes an outer pipe connecting the ends of one side of the plurality of thin-plate members; and an inner pipe provided inside the outer pipe, an outer pipe connecting the ends of one side of the plurality of thin-plate members; and an inner pipe provided inside the outer pipe, wherein the inner pipe has an inlet and a plurality of outlets, the refrigerant flowing into the inlet and being discharged from the plurality of outlets to the outer pipe, wherein the outer pipe has two closed ends, and a tube that forms a refrigerant inlet extends through a side wall of the outer pipe, and wherein the plurality of outlets are provided only in a lower part of the inner pipe in the direction of gravity.