Provided is a controller for use with a gas turbine engine, which ensures a stable combustion regardless of the deterioration of the catalyst and elevated characteristics of the exhaust gas. The controller 6 is used in the engine which comprises a combustor 2 for combusting a mixture of compressed air from a compressor 1 and a fuel under the existence of catalyst, the combustor having a catalytic combustion unit 21 bearing the catalyst and a pre-burner 7 provided on an upstream side of the catalytic combustion unit with respect to a flow of the compressed air for supplying and combusting a pre-heating fuel PF with the compressed air CA. The controller comprises a memory 6c for memorizing an initial temperature difference D between inlet and outlet temperatures t1, t2 measured at inlet and outlet of the catalytic combustion unit with non-deteriorated catalyst accommodated therein, and a pre-burner control for calculating a present temperature difference between the inlet and outlet temperatures measured in an operation of the gas turbine engine and controlling an amount of the fuel to be supplied to the pre-burner according to a deterioration Δd of the catalyst which is provided by a difference between the initial and present temperature differences D and d.