PURPOSE: To improve the filtration efficiency, prevent the clogging of a filter, facilitate the disposal of sludge, prevent the generation of sizing cake and reduce the size of apparatus by automatically cleaning a filter medium without stopping the sizing operation.

CONSTITUTION: A closed case 20 for filtration is placed in the middle of a sizing liquid pipe 16 for supplying a sizing liquid W from a cavity box 10 to a size box 11 and the inside of the case is divided into a stock liquid chamber and a filtration chamber with a cleaning screen 21. The stock liquid chamber is connected to the cavity box and the opposite filtration chamber is connected to the size box. The stock liquid chamber is provided with a bar brush 25 for scraping off the sludge on the cleaning screen 21. The lower end of the stock liquid chamber is connected to the upper part of the cavity box 10 through a return pipe 26 and a bag filter 28 attached to an end of the return pipe is immersed in the sizing liquid W in the cavity box.