PURPOSE: To prevent the levitating and scattering phenomena of coating mist in a coating booth, to improve work environment and to prevent coating failure by making the passage area of air flow on a water vessel in the coating booth gradually small according to the proceeding direction of the air flow.

CONSTITUTION: The air flow directed to a discharge port 19 from a feed port 18 is caused by a coating booth and coating for a material 15 to be coated which is positioned on a water vessel 16 is performed in this air flow e.g., by utilizing a coating gun 22. At this time, since the passage area of the air flow on the water vessel 16 is made small according to the proceeding direction of the air flow, the velocity of this air flow is made large in comparison with the conventional velocity and also the velocity is made large as the air flow is progressed forward. Therefore both the vapor of an solvent and coating mist which are generated by coating are accompanied with the air flow and surely sent to the discharge port 19 and discharged. Therefor the slash phenomena of coating mist are not caused and the levitating and scattering phenomena of coating mist in the coating booth can be prevented and work environment can be improved and also coating mist can be prevented from being stuck on both the uncoated material and the already coated material positioned in the coating booth.