To reduce the cost of parts and improve positioning precision by simplifying the shapes of parts and the structure of an air supply system.

A straight pipe unit P having an air supply port 4 on one end and an air discharge port 5 on the other, is disposed perpendicularly to the axial direction of a filter-element 3. A first opening 6 located in the middle of the straight pipe unit P communicates with the interior of the filter-element 3 through a pipe section 7, while an internal space formed between the first opening 6 and the air supply port 4 is blocked by a blocking part 8, a second opening 9 disposed between the blocking part 8 and the air supply port 4 communicates with the outside of the filter-element 3.

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