To suppress generation of SO2 to be of concern after ignition of a gasifying fuel, by keeping at a specified value air ratio of a gasifying agent comprising oxygen and air supplied to a gasification furnace during the period of time from an ignition of the gasifying fuel to a constant level of consumed load of the gasifying fuel.

Temperature and pressure are raised by increasing supply of starting fuel and air flow after the starting fuel is ignited. After temperature raise and pressure raise are completed, switching from the starting fuel to the gasifying fuel is performed so as to transfer to a gasifying state. At this moment, air ratio is made not higher than 1.0 during the period of time from an ignition of the fuel to constant level of load of the gasifying fuel by reducing air flow at the time of the ignition of the gasifying fuel. Thus, generation of SO2 to be of concern after ignition of the gassifying fuel can be suppressed, and piping and the like are prevented from undergoing low temperature corrosion.