Ammonia off-gas, which is separated and recovered by an ammonia recovery device from gasification gas produced in a gasification furnace in a two-column gasification system, is injected as reducing agent for denitration into an inlet of a cyclone for separation of bed material heated in a combustion furnace from combustion exhaust gas and for returning the bed material to the gasification furnace. The ammonia off-gas is contacted with the high-temperatured combustion exhaust gas to reduce and remove NOx through non-catalytic decomposition. Any excess of the ammonia off-gas is distributed for burning to primary air to the combustion furnace.