A refrigerant cycle apparatus comprising: at least a compressor, a radiator, decompression means capable of changing an open degree, a heat absorber, an internal heat exchanger that performs heat exchange between a refrigerant at an outlet of the radiator and the refrigerant at an outlet of the heat absorber, wherein first refrigerant conditions detection means for detecting standard conditions of at least the radiator and second refrigerant conditions detection means for detecting refrigerant conditions between an outlet of the radiator and a high-pressure side inlet of the internal heat exchanger are provided, and an opening of the decompression means is controlled so that a calculation value calculated based on at least an output of the first refrigerant conditions detection means and the output of the second refrigerant conditions detection means becomes a target value characterized in that the refrigerant cycle apparatus comprises: third temperature detection means for detecting an inlet temperature of a medium to be heated and fourth temperature detection means for detecting an outlet temperature of the medium to be heated, wherein the opening degree of the decompression means is controlled such that a calculation value calculated based on outputs of the first refrigerant condition detection means, the second refrigerant condition detection means, the third temperature detection means, and the fourth temperature detection means become a target value. at least a compressor, a radiator, decompression means capable of changing an open degree, a heat absorber, an internal heat exchanger that performs heat exchange between a refrigerant at an outlet of the radiator and the refrigerant at an outlet of the heat absorber, wherein first refrigerant conditions detection means for detecting standard conditions of at least the radiator and second refrigerant conditions detection means for detecting refrigerant conditions between an outlet of the radiator and a high-pressure side inlet of the internal heat exchanger are provided, and an opening of the decompression means is controlled so that a calculation value calculated based on at least an output of the first refrigerant conditions detection means and the output of the second refrigerant conditions detection means becomes a target value characterized in that the refrigerant cycle apparatus comprises: third temperature detection means for detecting an inlet temperature of a medium to be heated and fourth temperature detection means for detecting an outlet temperature of the medium to be heated, wherein the opening degree of the decompression means is controlled such that a calculation value calculated based on outputs of the first refrigerant condition detection means, the second refrigerant condition detection means, the third temperature detection means, and the fourth temperature detection means become a target value. third temperature detection means for detecting an inlet temperature of a medium to be heated and fourth temperature detection means for detecting an outlet temperature of the medium to be heated, wherein the opening degree of the decompression means is controlled such that a calculation value calculated based on outputs of the first refrigerant condition detection means, the second refrigerant condition detection means, the third temperature detection means, and the fourth temperature detection means become a target value.