To provide a combustion device and its operation method for sweeping away environmental issues and energy issues such as global warming and air pollution, factors of instability in supplying fossil fuel and the like, and its depletion.

This combustion device uses a means for burning hydrogen by a combustion device 4 of hydrogen fuel, generating steam by heat by the combustion to operate a steam turbine 1, electrolyzing the water in a water electrolyzing device 3 by electricity generated by a power generator 2 by the motive power of the steam turbine, and utilizing obtained hydrogen and oxygen as fuel of the combustion device 4 of hydrogen fuel and an oxidant of hydrogen. As the combustion device is operated by decomposition and combination of the water, the issues can be swept away.