PURPOSE: To enable a screen cleaning operation to be carried out easily and positively by a method wherein a box for receiving cleaning balls is arranged to be projected out of a casing.

CONSTITUTION: A funnel-like member enclosing around a hole 13 formed at an inner surface of a casing 1, projected out of the hole 13 outwardly and becoming tapered is installed to act as a receiving box 3 and then its extremity end is applied as an extraction port 12. When some cleaning balls are collected by a screen 2 and taken out, linear edges 24 are contacted to each other, an edge 25 positioned at a semi-ellipse longitudinal axis end is contacted with the inner surface of the casing 1 and two screens 2 are cooperatively arranged to close a passage within the easing 1. Accordingly, the cleansing balls floated within fluid are stopped at the screen 2, drop along the screen 2, are collected from the hole 13 into the receiving box 3 and finally taken out of the extraction port 12. With such an arrangement, the receiving box 3 is arranged outside the casing 1, so that the receiving box may not receive any resistance of fluid.