To reduce the amount of exhaust gas to be treated when the volume of waste is reduced.

Waste X is heated at ≤1,000°C in a low-oxygen atmosphere while steam S1 is made to coexist in a gasifying unit 6 to generate a mixed gas G1 while nitrogen oxides, sulfur oxides and dioxins are restrained from being generated. After halogen elements, metallic elements and compounds of them contained in the gas G1 are removed by a high-temperature filter 7, the filter-passed gas G1 is heated at 800-1,500°C in a low-oxygen atmosphere while steam S2 is made to coexist in a reformer 9 so that low molecular weight hydrocarbon in the gas G1 is decomposed into hydrogen, carbon monoxide and methane.