A two-stage compression refrigeration system with a refrigerant circuit comprising: a two-stage compressor that includes a low-stage compressor at a lower portion, a high-stage compressor at an upper portion and an electric motor at an intermediate portion, in a closed housing, the two-stage compressor being configured to discharge an intermediate-pressure refrigerant compressed by the low-stage compressor into the closed housing, sucking and compressing the intermediate-pressure refrigerant by the high-stage compressor, and then discharging the resulting refrigerant; an oil separating device that is provided on a discharge pipe from the two-stage compressor and that is configured to separate oil contained in compressed refrigerant gas; an oil return pipe configured to return the oil separated by the oil separating device, into the closed housing under an intermediate pressure; a gas cooler configured to cool the refrigerant gas after the oil is separated by the oil separating device, a gas-liquid separating device configured to separate the refrigerant cooled by the gas cooler into gas and liquid, or an internal heat exchanger configured to partially split the refrigerant cooled by the gas cooler to supercool, by the split refrigerant, a refrigerant flowing through the refrigerant circuit, and a gas injection pipe that is configured to inject an intermediate-pressure refrigerant gas from the gas-liquid separating device or from the internal heat exchanger into the closed housing of the two-stage compressor, the gas-liquid separating device or the internal heat exchanger constituting an economizer that is provided downstream of the gas cooler on the refrigerant circuit, characterized in that the oil return pipe and the gas injection pipe individually communicate with a space portion between the low-stage compressor and the electric motor in the closed housing. a two-stage compressor that includes a low-stage compressor at a lower portion, a high-stage compressor at an upper portion and an electric motor at an intermediate portion, in a closed housing, the two-stage compressor being configured to discharge an intermediate-pressure refrigerant compressed by the low-stage compressor into the closed housing, sucking and compressing the intermediate-pressure refrigerant by the high-stage compressor, and then discharging the resulting refrigerant; an oil separating device that is provided on a discharge pipe from the two-stage compressor and that is configured to separate oil contained in compressed refrigerant gas; an oil return pipe configured to return the oil separated by the oil separating device, into the closed housing under an intermediate pressure; a gas cooler configured to cool the refrigerant gas after the oil is separated by the oil separating device, a gas-liquid separating device configured to separate the refrigerant cooled by the gas cooler into gas and liquid, or an internal heat exchanger configured to partially split the refrigerant cooled by the gas cooler to supercool, by the split refrigerant, a refrigerant flowing through the refrigerant circuit, and a gas injection pipe that is configured to inject an intermediate-pressure refrigerant gas from the gas-liquid separating device or from the internal heat exchanger into the closed housing of the two-stage compressor, the gas-liquid separating device or the internal heat exchanger constituting an economizer that is provided downstream of the gas cooler on the refrigerant circuit, characterized in that the oil return pipe and the gas injection pipe individually communicate with a space portion between the low-stage compressor and the electric motor in the closed housing.