Plurality of fuel nozzles X1 and X2 in combustor X are supplied with fuel gas from fuel gas system and fuel oil from fuel oil system, respectively. Gas turbine operation is done with fuel being changed over to either gas or oil. Fuel oil distribution control system A controls oil flowing into plurality of fuel pipings. When oil is changed over to gas, fuel oil purge system B is supplied with air of appropriate temperature and pressure from purging air supply system C. This air flows into fuel oil pipings and nozzles X2 for purging residual oil therein. Fuel nozzle wash system D is supplied with water by-passing from wash water tank for compressor washing. This water flows through nozzles X2 for washing thereof.