To prevent steam pressure rise at power failure in an exhaust heat boiler for generating steam by an exhaust gas of a power generation device operating even at the time of power failure.

This exhaust heat boiler for a cogeneration system constituted by a power generation device 2 operating even at the time of power failure and the exhaust heat boiler 1 for generating steam by the exhaust gas from the power generation device 2 is provided with a steam releasing device 7 for releasing the steam when electricity application to the exhaust heat boiler 1 (exhaust heat boiler control device 8) is stopped. Further, an application time closed type solenoid valve is used as the steam releasing device.