PURPOSE: To obtain a constant catalyst combustion temperature by advancing and retreating a pre-combustion burner to change the amount of combustion air and to prevent blow off by providing the pre-combustion burner with a holder.

CONSTITUTION: A cylindrical casing 1 is provided with an inner tube 2 concentrically at the inside thereof and the inner tube 2 is arranged with the catalyst 9 at the intermediate part in the inner tube 2 while compressed air for combustion is passed through a path 14 between the casing 1 and the inner tube 2 to flow it into the inner tube 2 from the opening 15 of the end face of the inner tube. In such catalyst burner, the inner tube 2 is provided with a bypass air port 10 at the downstream side of the catalyst and a fuel nozzle 12 is opened at the upstream side of the same. The pre-combustion burner 5, consisting of a tubular holder 4 and a conical section 4a, is provided at the end in the inner tube and the conical section 4a is advanced or retreated in the opening 15 to change the amount of inflow air. As a result, the catalyst combustion temperature will never be changed even in case the output of a turbine is fluctuated and blow off is eliminated even in case of change of the flow amount of air whereby the stabilized pre-combustion may be maintained.