PURPOSE: To reduce the number of component parts, simplity pipings, and save a necessary space by using a cooling air system and a fuel oil atomizing air system of a gas turbine commonly partly.

CONSTITUTION: Discharge air from an air compressor 1 of a gas turbine 2 passes through a middle cooling means 4 and an air filtratiion means 5 till part of the air is guided to cooling air intake ports 2a, 2b. Part of the air passing the air filtration means 5 is branched at a branch point 7a to be compressed by a compressor 11 for atomizing air, and it is sent to a fuel nozzle 14 of a combustor 3 to atomize fuel oil guided from a pipe 16. For operation burning fuel gas instead of using fuel oil, fuel gas is supplied from a pipe 25 instead of the fuel oil, a valve 13 is changed to be full open, and air coming out of the compressor 11 passes through a conduit 30 to be returned to the upstream side of a middle cooling means 4 without going to the fuel nozzle 14. An air cooling means may be added between the branch point 7a and the atomizing air compressor 11.