PURPOSE: To reduce the generation of noise and to reduce the size of a blower by a method wherein a plurality of draft circuits each having an air intake port and an air discharge port are provided, a space between the draft circuits is partitioned by means of a reciprocative wall, and a damper is mounted to each opening.

CONSTITUTION: When a moving wall 4 is moved to a position, shown by a solid line, through the working of a drive mechanism 5, along with movement of air, a plurality of intake dampers 2 and 7 and discharge dampers 3 and 8 are moved to a position shown by a solid line. The internal volume of a first draft circuit 1 is decreased. Air in an amount equivalent to a reduced volume is moved through a first air discharge port 10 to the outside. Simultaneously, the internal volume of a second draft circuit 6 is increased. In which case, air in an amount equivalent to the increase of the volume is moved through a second air intake port 11 to the interior. Meanwhile, when the moving wall 4 is moved to a position shown by a broke line, through a reverse action to the above, the internal volume of the first draft circuit 1 is increased and the internal volume of the second draft circuit 6 is decreased.