To provide a cogeneration system capable of regularly performing desired hot water supply and heating regardless of a variation in the season, a time or the like.

The cogeneration system comprises a Rankine cycle including a first evaporator capable of evaporating a working medium, an expansion machine expanding the evaporated working medium, a first condenser condensing the working medium from the expansion machine, and a pump feeding the working medium from the first condenser to the first evaporator in which a generator is connected to the expansion machine so that the expanding work of the working medium can be taken as power, and heat radiation in the first condenser is usable as a heat source. This system further comprises a compressor compressing the working medium evaporated in a second evaporator, a second condenser condensing the working medium from the compressor, and a heat pump cycle returning the working medium from the second condenser to the second evaporator through a throttle mechanism, and the system is adapted so that the working media in the first condenser and the second evaporator are heat-exchangeable.