as shown in fig2 in schematic view the boundary between the first filter 53a and the second filter 53b the mesh a of the first filter 53b is divided by the mesh b of the second filter 53b which is smaller than the mesh a a part of the mesh b of the second filter 53b is divided by the mesh a of the first filter 53a therefore the leveled value of the size of the mesh at the boundary between the first filter 53a and the second filter 53b becomes smaller than the size of the second filter 53b and then a lot of the dust having the size smaller than the mesh b of the second filter 53b may be caught at the boundary as the result the substantive size of the mesh at the boundary becomes relatively smaller than the size of the mesh b of the second filter 53b the boundary between the final filter the third filter 53c in this embodiment and the filter 53b which is positioned prior to the final filter the substantive size of the mesh is determined according to a number of layers of the filters 53a 53b and 53c and each of the sizes of the meshes a b and c and then a limit diameter of dust.