A refrigeration cycle device comprising: a main compressor that compresses a refrigerant from a low pressure to a high pressure; a radiator that dissipates heat of the refrigerant, which has been discharged from the main compressor; an expander that reduces a pressure of the refrigerant, which has passed through the radiator; an evaporator that causes the refrigerant, which has flowed out from the expander, to evaporate; a sub-compression passage having one end connected to a suction pipe, which connects the evaporator with a suction side of the main compressor, and the other end connected to an intermediate position of a compression process of the main compressor; a sub-compressor that is provided in the sub-compression passage, compresses a part of the refrigerant with the low pressure, the part which has flowed out from the evaporator, to an intermediate pressure, and injects the refrigerant to the intermediate position of the compression process of the main compressor; and a driving shaft that connects the expander with the sub-compressor, and transfers power, which is generated when the pressure of the refrigerant is reduced by the expander, to the sub-compressor, wherein a design volume ratio (VC/VE), which is a value obtained by dividing a stroke volume VC of the sub-compressor by a stroke volume VE of the expander, is set to be smaller than (DE/DC)×(hE - hF)/(hB - hA), where, under a condition with an operating efficiency being the maximum in an operating range allowed to be set of the refrigeration cycle device, DE is a density of the refrigerant, which has flowed out from the radiator, DC is a density of the refrigerant, which has flowed out from the evaporator, hE is a specific enthalpy of the refrigerant, which flows into the expander, hF is a specific enthalpy of the refrigerant, which has flowed out from the expander, hA is a specific enthalpy of the refrigerant, which is sucked by the main compressor, and hB is a specific enthalpy of the refrigerant at the intermediate position of the compression process of the main compressor. a main compressor that compresses a refrigerant from a low pressure to a high pressure; a radiator that dissipates heat of the refrigerant, which has been discharged from the main compressor; an expander that reduces a pressure of the refrigerant, which has passed through the radiator; an evaporator that causes the refrigerant, which has flowed out from the expander, to evaporate; a sub-compression passage having one end connected to a suction pipe, which connects the evaporator with a suction side of the main compressor, and the other end connected to an intermediate position of a compression process of the main compressor; a sub-compressor that is provided in the sub-compression passage, compresses a part of the refrigerant with the low pressure, the part which has flowed out from the evaporator, to an intermediate pressure, and injects the refrigerant to the intermediate position of the compression process of the main compressor; and a driving shaft that connects the expander with the sub-compressor, and transfers power, which is generated when the pressure of the refrigerant is reduced by the expander, to the sub-compressor, wherein a design volume ratio (VC/VE), which is a value obtained by dividing a stroke volume VC of the sub-compressor by a stroke volume VE of the expander, is set to be smaller than (DE/DC)×(hE - hF)/(hB - hA), where, under a condition with an operating efficiency being the maximum in an operating range allowed to be set of the refrigeration cycle device, DE is a density of the refrigerant, which has flowed out from the radiator, DC is a density of the refrigerant, which has flowed out from the evaporator, hE is a specific enthalpy of the refrigerant, which flows into the expander, hF is a specific enthalpy of the refrigerant, which has flowed out from the expander, hA is a specific enthalpy of the refrigerant, which is sucked by the main compressor, and hB is a specific enthalpy of the refrigerant at the intermediate position of the compression process of the main compressor.