hot wire 12b in the hot wire anemometer 12 to obtain the flow angle of fluid velocity u03b8 a memory 15 which stores standard flow angle data a comparator 16 which compares the standard flow angle values from the memory 15 with the detected flow angle values from the flow angle processor 14 to obtain the difference between them a phase difference value obtained circuit 17 which produces a phase difference for the difference from the comparator 16 to compensate the positional delay and the fluid inertial delay due to the setting position interval between the baffle vanes 5 and the hot wire anemometers 12 a reversing circuit 18 which changes the sign of the difference value obtained from the phase difference circuit 17 a memory for standard 19 which stores the standard angle data for the baffle vanes 5 an adder 20 which adds the standard angle for baffle vanes from the memory 19 to the difference value obtained from the reversing circuit 18 and a subtracter 21 the signals for controlling the baffle vanes vane angles as received from the adder 20 are supplied to the actuator 11 through the subtracter 21 the subtracter 21 receives the angle signals as a negative feedback from a position detector 22 installed in the actuator 11 the memory 15 for standard flow angle data as described above is set to store the standard flow angle value.