A fuel-air premixing device of a gas turbine has a double-cylinder structure composed of an inner cylinder and an outer cylinder defining therebetween flow passages such that a fuel is supplied into air flowing through the passages so that a pre-mixture of the fuel and air is formed and discharged. The premixing device further has a plurality of radially spaced fuel nozzles capable of supplying the fuel into the passages, and fuel supply rate regulator valves for independently controlling the rates of supply of the fuel from the radially spaced nozzles.