PURPOSE: To enable to wash a cake with a small amount of water and contrive to recover valuable components or to enhance the purity of a product, by a method wherein a filter cloth is lifted up at a part thereof to provide a washing space between the filter cloth and a cake.

CONSTITUTION: The material to be treated is charged into a cake guide frame 29 from a supplying device, is placed on a lower filter cloth 11 and is brought to a tension roll 5. In this region, gravity dehydration for removing free water is performed. Then, the cake is gradually pressed between the upper and lower filter cloths 10, 11 to accomplish a primary dehydration. Subsequently, the upper filter cloth 10 is lifted up by a take-up roll 18 to temporarily return the cake to normal pressure, and the cake is washed by spraying washing water from a washing sprayer 32. The cake thus washed is clamped between the upper and lower filter cloths 10, 11, is passed through wedge rolls and tension rolls to perform a secondary pressure dehydration, then it is brought to driving rolls 8, 9 and is discharged by scraping off by a scraper.