To provide a cogeneration system capable of efficiently accumulating heat.

A hot water storage tank 20 and a heat exchanger 14 of a cogeneration device 10 are connected to each other by an outbound passage 31 and a return passage 32, and a feed pump 30 is provided on the outbound passage 31. A control device 40 (control means) selects and executes either a first heat accumulation mode to adjust an output of the feed pump 30 to set a difference between the detected water temperature by an outbound passage temperature sensor T31 and a return passage temperature sensor T32 at a prescribed value or a second heat accumulating mode to adjust an output of the feed pump 30 to set the detected water temperature by the return passage temperature sensor T32 at a prescribed value in accordance with the detected water temperature by the outbound passage temperature sensor T31.