A thermal energy recovery device comprising: an evaporator configured to evaporate a mixed medium including a working medium and an oil; an oil separator configured to separate the oil from the mixed medium discharged from the evaporator; an expander configured to expand the working medium discharged from the oil separator; a power recovery machine connected to the expander; an oil supply flow passage configured to supply the oil, discharged from the oil separator, to the expander; a condenser configured to condense the mixed medium discharged from the expander; a pump configured to send the mixed medium, discharged from the condenser, to the evaporator; a circulation flow passage connecting the evaporator, the oil separator, the expander, the condenser, and the pump in the mentioned order; an oil cooler configured to cool the oil flowing through the oil supply flow passage; an oil cooling flow passage branched from a portion between the pump and the evaporator in the circulation flow passage and configured to pass the oil cooler; and an expansion valve provided at a portion upstream of the oil cooler in the oil cooling flow passage and configured to adjust the opening; characterized by further comprising an expansion valve adjustment portion configured to adjust the opening of the expansion valve so that the temperature of the oil flowing through a portion between the oil cooler and the expander in the oil supply flow passage is maintained at a value below a reference temperature, wherein the oil cooler allows the oil flowing through the oil supply flow passage to be thermally exchanged with the mixed medium flowing through the oil cooling flow passage and evaporates at least a portion of the mixed medium, and a downstream end of the oil cooling flow passage is connected to a middle step of the expander. an evaporator configured to evaporate a mixed medium including a working medium and an oil; an oil separator configured to separate the oil from the mixed medium discharged from the evaporator; an expander configured to expand the working medium discharged from the oil separator; a power recovery machine connected to the expander; an oil supply flow passage configured to supply the oil, discharged from the oil separator, to the expander; a condenser configured to condense the mixed medium discharged from the expander; a pump configured to send the mixed medium, discharged from the condenser, to the evaporator; a circulation flow passage connecting the evaporator, the oil separator, the expander, the condenser, and the pump in the mentioned order; an oil cooler configured to cool the oil flowing through the oil supply flow passage; an oil cooling flow passage branched from a portion between the pump and the evaporator in the circulation flow passage and configured to pass the oil cooler; and an expansion valve provided at a portion upstream of the oil cooler in the oil cooling flow passage and configured to adjust the opening; characterized by further comprising an expansion valve adjustment portion configured to adjust the opening of the expansion valve so that the temperature of the oil flowing through a portion between the oil cooler and the expander in the oil supply flow passage is maintained at a value below a reference temperature, wherein the oil cooler allows the oil flowing through the oil supply flow passage to be thermally exchanged with the mixed medium flowing through the oil cooling flow passage and evaporates at least a portion of the mixed medium, and a downstream end of the oil cooling flow passage is connected to a middle step of the expander.