A light portion is extracted from feed oil by a separation system. The light portion is then subject to a hydrotreatment to obtain impurity-removed fuel oil which is stored in an intermediate tank. A residue of the feed oil after extraction of the light portion is gasified to obtain syngas (H2 gas+CO gas) which is used as basic fuel for power generation in a power generation system. The fuel oil is fed to the power generation system as auxiliary fuel for supplementing the power generation based on the syngas. The power generation system includes a plurality of gas turbines and generators. The number of the gas turbines to be driven by the fuel oil is controlled to adjust the power generation amount depending on demand.