PURPOSE: To enable to set the most proper air volume easily by a method wherein measuring pipes, communicating with atmosphere, are provided at each air inducing points and the opening degrees of each induced air volume adjusting dampers are regulated based on the representative value measured by the measuring pipe.

CONSTITUTION: The necessary passing air volumes V1WV6 of the measuring pipes 4, provided at each air inducing points AWF, are corresponding to each induced air volume adjusting dampers (a)W(f). The air inducing point A is the first fixed air inducing point and the opening degree of the damper (a) is limited, then, the passing air volume V11 through the measuring pipe at the air inducing point A is obtained. Next, the air volume V21 passing through the measuring pipe at the air inducing point B is obtained and the damper (b) is regulated so that the air volume V21 becomes equal to V11×(V2/V1). According to this regulation, an air volume distributing ratio from a joining point (j) between the joining duct of the air inducing points A, B and the joining duct of the air inducing points C, D to the air inducing points A, B may be obtained.