A thermal energy recovery device includes a circulation flow path for circulating a working fluid, a thermal fluid circulation flow path for circulating hot water, an evaporator for evaporating the working fluid flowing in the circulation flow path by heat of the hot water flowing in the thermal fluid circulation flow path, a preheater for heating the working fluid before flowing into the evaporator by the heat of the hot water flowing in the thermal fluid circulation flow path, and a control unit for controlling a startup operation of the thermal energy recovery device. The control unit executes a suppression control for suppressing a temperature difference between the hot water and the working fluid in the preheater.