Abstract: A cogeneration apparatus is arranged to properly respond to a plurality of separate demands for supplying the thermal energy. A hot water storage tank 17 is provided for storing a first hot water produced using waste heat from an engine generator 10. A first heat exchanger 20 for producing the first hot water and a second heat exchanger 22 for producing a second hot water by drawing heat from the first hot water are provided in the hot water storage tank 17. A temperature sensor TS1 is provided between the first heat exchanger 20 and the second heat exchanger 22 while a second temperature sensor TS2 is provided above the second heat exchanger 22. A controller 29 control the operation of the engine generator 10 in response to the conditions of thermal loads 21 and 24 determined by the measurements of temperature detected by the temperature sensors TS1 and TS2. Also, a re-heating boiler 25 is provided for heating the second hot water to be supplied to the thermal load 24.