PURPOSE: To adjust a blow-off air speed under a constant blow-off airflow amount by a method wherein second blade board is supported between the first blade boards so as to be moved reciprocally into blow-off direction and change the sectional area of the opening of the blow-off port formed between the first blade boards.

CONSTITUTION: When the second blade board 18 is pushed into the blow-off port, a link member 21 pushes the blow-off side end parts 13, 16 of the first blade board 10 toward the outside of the blow-off port 1, while the blow-off side end parts 13, 16 are turned outwardly about the centers of supporting shafts 15, 12 to maximize a clearance C and thereby maximizing the sectional area of the opening of the blow-off port. When the second blade board 18 is withdrawn toward the blow-off side thereof, the link member 21 pulls the blow-off side end parts 13, 16 toward the inside of the blow-off port 1 while the blow-off side end parts 13, 16 are turned inwardly about the supporting shafts 15, 12 to decrease the clearance C, the aft end part 23 of the first blade board 10 is turned outwardly about the supporting shafts 15, 12 to abut against an inner wall surface 24 and thereby decreasing the sectional area of the opening.