and the outer side of the first cylindrical screen 1 and from the portion located at the other end surface so that it is possible to solve the u201c inconvenience that concentration is increased as the paper stock goes downstream of the screen u201d further the connection member 4 which is a ring member is in parallel to the one of end surfaces and the other end surface of the second cylindrical screen 2 and is located substantially at an intermediate portion between the one end surface and the other end surface the connection member 4 includes the wall 43 swelling outward therefore paper stock having great amount of foreign matters which can not pass through the first cylindrical screen 1 and the second cylindrical screen 2 is effectively collected in the groove 42 formed over substantially the entire length of the inside of the ring member 4 and dust eliminating effect can be enhanced according to the first aspect of the present invention the selecting processing amount of the paper stock is increased by the first cylindrical screen and the second cylindrical screen provided in the tank the paper stock is introduced to the first cylindrical screen and the second cylindrical screen from the portion located at.