To remove a material, such as wet ash, sticking to the inside of a hopper with less labor.

A transport route (a water supply control valve 8, a drain gutter 9) different from an ash transport pipe 15 is installed. The wet ash sticking to an ash hopper 2 is washed with hot water injected from a washing water nozzle 7 and polluted water after the wash is flushed to the drain gutter 9 through the water supply control valve 8. Thereby, the set ash sticking to the ash hopper 2 can be surely removed, and the necessity for worker's operation of scraping out the wet ash from the ash hopper 2 can be eliminated to drastically reduce the work burden on the worker.