A heat pump apparatus comprising: a refrigerant circuit configured to circulate refrigerant having flammability; a heat medium circuit configured to allow a heat medium to flow therethrough; a heat-medium heat exchanger configured to exchange heat between the refrigerant and the heat medium; an outdoor unit configured to accommodate the refrigerant circuit; and an indoor unit configured to accommodate a part of the heat medium circuit, a refrigerant release valve being at least one of a pressure relief valve and an air purge valve which are provided in the heat medium circuit, the outdoor unit having a first chamber in which at least a compressor of the refrigerant circuit is provided, wherein the outdoor unit comprises the heat-medium heat exchanger and the refrigerant release valve downstream of the heat-medium heat exchanger in a flow direction of the heat medium in a same chamber, wherein the first chamber further comprises an electrical component box configured to accommodate a control board constituting a controller, and a relay configured to switch supply and interruption of power to the compressor and an outdoor air-blowing fan configured to blow the outdoor air to the air heat exchanger characterized in that (a) the refrigerant release valve is provided in the first chamber at a position below the electrical component box, the first chamber has a first ventilation port formed above the refrigerant release valve, and a second ventilation port formed below the refrigerant release valve and the refrigerant has a density larger than air density under an atmospheric pressure, or, (b) the outdoor unit further has a second chamber (131, 133, 122) partitioned from the first chamber, the refrigerant release valve being provided in the second chamber (131, 133, 122). a refrigerant circuit configured to circulate refrigerant having flammability; a heat medium circuit configured to allow a heat medium to flow therethrough; a heat-medium heat exchanger configured to exchange heat between the refrigerant and the heat medium; an outdoor unit configured to accommodate the refrigerant circuit; and an indoor unit configured to accommodate a part of the heat medium circuit, a refrigerant release valve being at least one of a pressure relief valve and an air purge valve which are provided in the heat medium circuit, the outdoor unit having a first chamber in which at least a compressor of the refrigerant circuit is provided, wherein the outdoor unit comprises the heat-medium heat exchanger and the refrigerant release valve downstream of the heat-medium heat exchanger in a flow direction of the heat medium in a same chamber, wherein the first chamber further comprises an electrical component box configured to accommodate a control board constituting a controller, and a relay configured to switch supply and interruption of power to the compressor and an outdoor air-blowing fan configured to blow the outdoor air to the air heat exchanger characterized in that (a) the refrigerant release valve is provided in the first chamber at a position below the electrical component box, the first chamber has a first ventilation port formed above the refrigerant release valve, and a second ventilation port formed below the refrigerant release valve and the refrigerant has a density larger than air density under an atmospheric pressure, or, (b) the outdoor unit further has a second chamber (131, 133, 122) partitioned from the first chamber, the refrigerant release valve being provided in the second chamber (131, 133, 122).