In the present invention, in a cogeneration system that stores in a hot water tank hot water that has been heated by waste heat from a generator, while suppressing freezing of water in a water circulation pathway during the interval of recovering from freezing suppression control to normal control, normal control is recovered rapidly, suppressing a decrease in system efficiency. When freezing conditions are met at which there is the risk of water in a water circulation pathway at a radiator unit or the like will freeze, freezing suppression control is executed, and when conditions are met for cancelling freezing suppression control, the target value (first recovery target value) for the heat recovery temperature at the exit of the generator is set to a minimum target temperature (SVTf2_min), and thereafter, until the actual heat recovery exit temperature (actual temperature) (PVTf2) reaches a standard target value (Tf2_std), each time the actual temperature (PVTf2) reaches a value resulting from dividing a predetermined temperature (x) by the first recovery target value (SVTf2), the first recovery target value (SVTf2) is updated to a value resulting from adding a predetermined temperature (y (>x)).