To facilitate handling and eliminate substantially the demand for maintenance and contrive to reduce cost by operating a door of a plurality of intake holes of a porous gate with each stage of operation rod provided with a connection relation and connecting to a lowermost stage in a chain.

A first stage door 2 is lifted up so that a stopper 51 of a second stage operation rod whose tip is fixed with the lower end 22 may reach the bottom of guide hardware 41 of a second stage door 4, thereby lifting up the second stage door 4 and opening a second stage intake hole 9. Furthermore, guide hardware 61 is lifted up with a third stage door 6, thereby opening a third intake hole 10. In the case when they are closed, they fall down with the self weight of the door by the reversed procedure, thereby closing the intake holes. This construction makes it possible to open and close freely the intake holes in simple structure by operating a single switch and thereby reduce load in the site.