PURPOSE: To make an inner sleeve or the like unnecessary to reduce the cost, by keeping the air pressure of the space between a gathering vessel and a bag lower than the air pressure of the inside of the bag.

CONSTITUTION: A by-pass pipe 26 has one end opened in a gathering vessel 8 and has the other end connected to a duct connected to a blower 12 and a secondary filter 11. A perforated inside cylinder of the gathering vessel 8 consists of a member having air-permeability and holds always a certain space between the bag 24 and the inside wall of the gathering vessel 8. When the pressure in the bag is denoted as P1 and the pressure of the space between the bag 24 and the gathering vessel 8 is denoted as P2, the pressure P1 is higher than the pressure P2 by a value corresponding to the pressure loss due to two filters 10 and 11 at least, that is, P1>P2 is true, and the bag 24 is expanded and is stuck to the inside of the inside cylinder 33. Thus, the inner sleeve for the countermeasure against negative pressure is unnecessary.