To obtain high power generation efficiency, utilizing waste solid fuel such as RDF as a part of fuel for power generation, with high utilization factor, and also to suppress the occurrence of a variety of dioxin while suppressing the corrosion of a heat conductive pipe or the like.

In a method of combusting waste solid fuel such as RDF, etc., in a waste solid fuel stoking boiler, the steam at 400°C or under generated in a waste solid fuel stoking boiler plant such as an RDF stoking boiler plant 50 or the like is used as at least one part of a heat source for the feed waste heater 54 and/or the fuel economizer 56 of a power generation plant 52 which generates power by combusting fossil fuel such as heavy oil, coal, or the like as the main fuel, thereby generating steam.