A refrigeration cycle device comprising: a compressor for compressing a gaseous refrigerant; a condenser for condensing the compressed refrigerant; a decompressor for decompressing the condensed refrigerant; an evaporator for evaporating the decompressed refrigerant; a subcooling device for subcooling the refrigerant condensed by the condenser; a liquid injection valve for adjusting part of the refrigerant after passing through the subcooling device to have a predetermined flow rate and for injecting it to an intermediate section of the compressing chamber of the compressor; an appropriate subcooling degree estimation section configured for estimating an appropriate subcooling degree of the refrigerant at an outlet of the subcooling device based on an estimated refrigerant circulation quantity (Grp) of the refrigerant circulating from a discharging side of the compressor through the condenser, and the subcooling device using equations Sc1s = A 1 · Ln Grp − A 2 Grp = a 1 · Fr + A 2 b 1 · Ps + b 2 c 1 · MV + c 2 wherein Sc1s is the appropriate subcooling degree, A1 and A2 are predetermined coefficients acquired from a relation between the refrigerant circulation quantity and the appropriate subcooling degree, Grp represents an estimated value of the refrigerant circulation quantity, Fr represents compressor rotation speed, Ps represents an inlet pressure of the compressor, MV is an opening degree of the liquid injection valve, and a1, a2, b1, b2, c1, and c2 are coefficients acquired by experiment or simulation, respectively; a determination threshold calculating section configured for calculating a determination threshold (Sc1th) of the subcooling degree based on the estimated appropriate subcooling degree using equation Sc1th = Sc1s × A3 wherein A3 is a positive integral lower than 1; and a determining process section configured for determining whether refrigerant leakage occurs by comparing a measured subcooling degree and the determination threshold, wherein the passage of the refrigerant flowing out from the subcooling device is divided into first and second passages; wherein during operation of the refrigeration cycle device, the refrigerant in the first passage is decompressed by a liquid injection decompressor, and then, injected into an intermediate pressure of the compressor; wherein, during operation of the refrigeration cycle device, the refrigerant in the second passage is supplied to the decompressor. a compressor for compressing a gaseous refrigerant; a condenser for condensing the compressed refrigerant; a decompressor for decompressing the condensed refrigerant; an evaporator for evaporating the decompressed refrigerant; a subcooling device for subcooling the refrigerant condensed by the condenser; a liquid injection valve for adjusting part of the refrigerant after passing through the subcooling device to have a predetermined flow rate and for injecting it to an intermediate section of the compressing chamber of the compressor; an appropriate subcooling degree estimation section configured for estimating an appropriate subcooling degree of the refrigerant at an outlet of the subcooling device based on an estimated refrigerant circulation quantity (Grp) of the refrigerant circulating from a discharging side of the compressor through the condenser, and the subcooling device using equations Sc1s = A 1 · Ln Grp − A 2 Grp = a 1 · Fr + A 2 b 1 · Ps + b 2 c 1 · MV + c 2 wherein Sc1s is the appropriate subcooling degree, A1 and A2 are predetermined coefficients acquired from a relation between the refrigerant circulation quantity and the appropriate subcooling degree, Grp represents an estimated value of the refrigerant circulation quantity, Fr represents compressor rotation speed, Ps represents an inlet pressure of the compressor, MV is an opening degree of the liquid injection valve, and a1, a2, b1, b2, c1, and c2 are coefficients acquired by experiment or simulation, respectively; a determination threshold calculating section configured for calculating a determination threshold (Sc1th) of the subcooling degree based on the estimated appropriate subcooling degree using equation Sc1th = Sc1s × A3 wherein A3 is a positive integral lower than 1; and a determining process section configured for determining whether refrigerant leakage occurs by comparing a measured subcooling degree and the determination threshold, wherein the passage of the refrigerant flowing out from the subcooling device is divided into first and second passages; wherein during operation of the refrigeration cycle device, the refrigerant in the first passage is decompressed by a liquid injection decompressor, and then, injected into an intermediate pressure of the compressor; wherein, during operation of the refrigeration cycle device, the refrigerant in the second passage is supplied to the decompressor.