To provide a gas turbine co-generation system efficiently utilizing a surplus biogas.

The surplus of power generated by a turbo generation unit 5 is stored in a battery 7, and when the output of the turbo generation unit cannot cope with a load 8, the power stored in the battery is imparted to the load. Besides, warm water or vapor is heated by using the waste heat discharged from the generation unit. Further, as the fuel for the generation unit, a biogas (a) which is obtained by fermenting a raw material such as the excrement of livestock or organic waste in a fermenter 1 is used, and the surplus biogas is stored in a pressurized gas holder 4.