This IGCC plant is provided with an ASU which separates oxygen gas and nitrogen gas from air, a coal gasification furnace which uses an oxidizing agent to gasify coal, and a gas turbine which is driven by the combustion gas resulting from burning a gas generated by means of the coal gasification furnace. This IGCC plant control device (50) is provided with an air separation amount determination unit (52) which determines the production amount of nitrogen gas produced by the ASU depending on the operating load of the IGCC plant, and supplies to the coal gasification furnace the entire amount of oxygen gas generated as a byproduct in response to the determined nitrogen gas production amount. By this means, the IGCC plant can minimize blow-off of oxygen gas produced from the air.