bottom plate connecting the two side plates together the rectangular shaped bottom plate has a width approximately equal to a width of the air inlet port 22 and a length greater than a vertical directional length of the air inlet port 22 further one of adjacent sides of the right triangle shaped side plate extending in the vertical direction has a length approximately equal to the vertical directional length of the air inlet port 22 the hood 21 is formed in a cross sectionally angular c shape by joining opposite lateral edges of the bottom plate to respective oblique sides of the two side plates arranged in parallel no component is disposed in a space between shorter adjacent sides of the two side plates thus an opening opened upwardly is formed between the shorter adjacent sides of the two side plates the secondary small chamber 17 b 18 b is communicated with the process chamber 13 b 13 a through the opening 22 a on the other hand as shown in fig5 the bottom plate of the hood 21 is disposed on the side of the secondary small chamber 17 b 18 b with respect to the air inlet port 22 in such a manner as to extend upwardly from a lower edge thereof connected to a lower edge of the air inlet port 22 while being gradually spaced away from the partition plate 12 b 12 a therefore the bottom plate of the hood 21 is disposed in such a manner as to be horizontally.