An apparatus for controlling a gas turbine combustor having a diffusion combustion burner and a premix combustion burner comprising: a rotating speed detector configured to detect a rotating speed of the gas turbine, a recorder configured to record the detected value of the rotating speed of gas turbine detected by the rotating speed detector, an arithmetic unit configured to calculate a change with time of the rotating speed of gas turbine in accordance with details of the detected value of the rotating speed of gas turbine recorded in the recorder, and a fuel control unit configured to judge a starting situation of reduction in the rotating speed of gas turbine on the basis of the change with time of the rotating speed of gas turbine calculated by the arithmetic unit and to control respectively a fuel flow rate for the diffusion combustion burner to be fed to the diffusion combustion burner installed in the gas turbine combustor and a fuel flow rate for the premix combustion to be fed to the premix combustion burner, characterized in that: the arithmetic unit configured to calculate the change with time of the rotating speed of gas turbine is structured so as to detect a situation that the rotating speed of gas turbine increased at the time of a load dump of the gas turbine starts to reduce by calculation of the change with time of the rotating speed of gas turbine, to output an operation signal for controlling the fuel flow rate to the fuel control unit, switch, by the fuel control unit, a combustion state of the gas turbine combustor from independent diffusion combustion by the diffusion combustion burner to mixed combustion of diffusion combustion and premix combustion by the diffusion combustion burner and the premix combustion burner, and to control respectively the fuel flow rate for the diffusion combustion burner fed to the diffusion combustion burner and the fuel flow rate for the premix combustion fed to the premix combustion burner. a rotating speed detector configured to detect a rotating speed of the gas turbine, a recorder configured to record the detected value of the rotating speed of gas turbine detected by the rotating speed detector, an arithmetic unit configured to calculate a change with time of the rotating speed of gas turbine in accordance with details of the detected value of the rotating speed of gas turbine recorded in the recorder, and a fuel control unit configured to judge a starting situation of reduction in the rotating speed of gas turbine on the basis of the change with time of the rotating speed of gas turbine calculated by the arithmetic unit and to control respectively a fuel flow rate for the diffusion combustion burner to be fed to the diffusion combustion burner installed in the gas turbine combustor and a fuel flow rate for the premix combustion to be fed to the premix combustion burner, characterized in that: the arithmetic unit configured to calculate the change with time of the rotating speed of gas turbine is structured so as to detect a situation that the rotating speed of gas turbine increased at the time of a load dump of the gas turbine starts to reduce by calculation of the change with time of the rotating speed of gas turbine, to output an operation signal for controlling the fuel flow rate to the fuel control unit, switch, by the fuel control unit, a combustion state of the gas turbine combustor from independent diffusion combustion by the diffusion combustion burner to mixed combustion of diffusion combustion and premix combustion by the diffusion combustion burner and the premix combustion burner, and to control respectively the fuel flow rate for the diffusion combustion burner fed to the diffusion combustion burner and the fuel flow rate for the premix combustion fed to the premix combustion burner. the arithmetic unit configured to calculate the change with time of the rotating speed of gas turbine is structured so as to detect a situation that the rotating speed of gas turbine increased at the time of a load dump of the gas turbine starts to reduce by calculation of the change with time of the rotating speed of gas turbine, to output an operation signal for controlling the fuel flow rate to the fuel control unit, switch, by the fuel control unit, a combustion state of the gas turbine combustor from independent diffusion combustion by the diffusion combustion burner to mixed combustion of diffusion combustion and premix combustion by the diffusion combustion burner and the premix combustion burner, and to control respectively the fuel flow rate for the diffusion combustion burner fed to the diffusion combustion burner and the fuel flow rate for the premix combustion fed to the premix combustion burner.