A screw expander system, comprising: a screw expander, including an expansion space formed by screw rotors meshed with each other, the expansion space for expanding an operating medium, an operating medium inlet and an operating medium outlet communicating with the expansion space, a bearing for supporting a rotor shaft of the screw rotor, an oil inlet (23, 24) for supplying oil to at least one of the expansion space and the bearing, and an oil outlet (25, 26) for discharging the oil from at least one of the expansion space and the bearing; a main body outlet side line connected to the operating medium outlet and the oil outlet (25, 26), the main body outlet side line for feeding the operating medium from the operating medium outlet and the oil from the oil outlet (25, 26); a condenser disposed in said main body outlet side line; a pump connected to said main body outlet side line, the pump for pressure-feeding a mixture of the operating medium and the oil fed by said main body outlet side line; and an evaporator for evaporating the operating medium in the mixture pressure-fed by said pump, wherein in the screw expander system for circulating and supplying the operating medium from said evaporator to the operating medium inlet, and circulating and supplying the oil from said evaporator to the oil inlet (23, 24), an oil separating tank for separating the operating medium and the oil is disposed between said evaporator and said screw expander, the oil separating tank having an operating medium containing section for containing the separated operating medium, and an oil containing section for containing the separated oil, and the oil containing section of said oil separating tank is connected to the oil inlet (23, 24) of said screw expander; characterized in that the oil inlet (23, 24) includes a first oil inlet communicating with the bearing, and a second oil inlet communicating with the expansion space, and a pressure adjusting means for adjusting a pressure on the upstream side and a pressure on the downstream side is disposed between the operating medium containing section of said oil separating tank and the operating medium inlet of said screw expander. a screw expander, including an expansion space formed by screw rotors meshed with each other, the expansion space for expanding an operating medium, an operating medium inlet and an operating medium outlet communicating with the expansion space, a bearing for supporting a rotor shaft of the screw rotor, an oil inlet (23, 24) for supplying oil to at least one of the expansion space and the bearing, and an oil outlet (25, 26) for discharging the oil from at least one of the expansion space and the bearing; a main body outlet side line connected to the operating medium outlet and the oil outlet (25, 26), the main body outlet side line for feeding the operating medium from the operating medium outlet and the oil from the oil outlet (25, 26); a condenser disposed in said main body outlet side line; a pump connected to said main body outlet side line, the pump for pressure-feeding a mixture of the operating medium and the oil fed by said main body outlet side line; and an evaporator for evaporating the operating medium in the mixture pressure-fed by said pump, wherein in the screw expander system for circulating and supplying the operating medium from said evaporator to the operating medium inlet, and circulating and supplying the oil from said evaporator to the oil inlet (23, 24), an oil separating tank for separating the operating medium and the oil is disposed between said evaporator and said screw expander, the oil separating tank having an operating medium containing section for containing the separated operating medium, and an oil containing section for containing the separated oil, and the oil containing section of said oil separating tank is connected to the oil inlet (23, 24) of said screw expander; characterized in that the oil inlet (23, 24) includes a first oil inlet communicating with the bearing, and a second oil inlet communicating with the expansion space, and a pressure adjusting means for adjusting a pressure on the upstream side and a pressure on the downstream side is disposed between the operating medium containing section of said oil separating tank and the operating medium inlet of said screw expander.