To provide a cogeneration system capable of changing timing for taking out electricity and heat and achieving efficient operation without wasting either of energy.

This cogeneration system drives a motor 1 such as an engine, a turbine or the like to generate power by a generator 2 and recovers and utilizes waste heat of exhaust gas of the motor 1. Exhaust gas of the motor 1 is exhausted by passing through a heat reserve tank 5 to reserve waste heat in the heat reserve tank 5 and recover waste heat. Preheated air obtained by flowing the air in the heat reserve tank 5 in a time zone when there is heat demand irrespective of driving the motor 1 is supplied to a boiler 3 to obtain steam or hot water.