A method for efficiently conveying impurities in a pressurized fluidized incinerator system is provided. Cleaning gas is supplied to an upper valve, and thereafter, the upper valve is driven so as to communicate an upper discharge device and a tank. The upper discharge device is driven so as to convey the impurities from the dust collector to the tank, and thereafter, the upper discharge device is stopped and the upper valve is driven so as not to communicate the upper discharge device and the tank. Thereafter, the supply of the cleaning gas to the upper valve is stopped.