A heat pump system comprising: a main refrigerant circuit formed by connecting a compressor for compressing a refrigerant; a radiator for radiating the refrigerant compressed in said compressor; a variable-type main depressurization mechanism for depressurizing the refrigerant radiated in said radiator; and an evaporator for evaporating the refrigerant depressurized in said main depressurization mechanism; a bypass circuit having a bypass tube for connecting an intake side of said compressor and a portion of said main refrigerant circuit that extends from the outlet of said radiator to said main depressurization mechanism; and a bypass depressurization mechanism for depressurizing the liquid refrigerant flowing through said bypass tube; and a controller for controlling said compressor, said main depressurization mechanism, and said bypass depressurization mechanism; wherein said bypass depressurization mechanism is a variable-type bypass depressurization mechanism; and said controller is configured to perform liquid injection control for controlling said bypass depressurization mechanism so that the discharge temperature (Td), which is the temperature of the refrigerant discharged from said compressor, is equal to a predetermined target discharge temperature (Tds); and the upper limit opening degree (OPix) of said bypass depressurization mechanism in said liquid injection control is determined by a correlation value with respect to the opening degree (OPe) of said main depressurization mechanism, characterized in that the compressor is a variable capacity compressor, and said controller is configured to change the opening degree of said bypass depressurization mechanism to a predicted opening degree (OPif), which is an opening degree predicted according to the degree of change in the capacity of said compressor, when the capacity of said compressor is changed during said liquid injection control. a main refrigerant circuit formed by connecting a compressor for compressing a refrigerant; a radiator for radiating the refrigerant compressed in said compressor; a variable-type main depressurization mechanism for depressurizing the refrigerant radiated in said radiator; and an evaporator for evaporating the refrigerant depressurized in said main depressurization mechanism; a bypass circuit having a bypass tube for connecting an intake side of said compressor and a portion of said main refrigerant circuit that extends from the outlet of said radiator to said main depressurization mechanism; and a bypass depressurization mechanism for depressurizing the liquid refrigerant flowing through said bypass tube; and a controller for controlling said compressor, said main depressurization mechanism, and said bypass depressurization mechanism; wherein said bypass depressurization mechanism is a variable-type bypass depressurization mechanism; and said controller is configured to perform liquid injection control for controlling said bypass depressurization mechanism so that the discharge temperature (Td), which is the temperature of the refrigerant discharged from said compressor, is equal to a predetermined target discharge temperature (Tds); and the upper limit opening degree (OPix) of said bypass depressurization mechanism in said liquid injection control is determined by a correlation value with respect to the opening degree (OPe) of said main depressurization mechanism, characterized in that the compressor is a variable capacity compressor, and said controller is configured to change the opening degree of said bypass depressurization mechanism to a predicted opening degree (OPif), which is an opening degree predicted according to the degree of change in the capacity of said compressor, when the capacity of said compressor is changed during said liquid injection control.