PURPOSE: To eliminate the clogging of filter media and cake with a simple operation and to prevent the trouble and danger owing to abnormal vibration, by providing a skimming pipe which can control the starting time and speed of filtration and a washing liquid pipe which can be backwashed.

CONSTITUTION: The position of the forward end port of a skimming pipe 9 is pulled up to the bore side from the height of a partition wall 6 by a control cylinder 10 prior to the supply of raw liquid with a supply pipe 12 for raw liquid. When the raw liquid is supplied thereafter, the filtrate is not skimmed and therefore the filtration is not accomplished quickly. The raw liquid is stagnated for a specified time as a raw liquid layer 17 in a rotating body, and when a crystal settles uniformly on a filter cloth 5, the pipe 9 is sunck in a partition chamber 7. On the other hand, when a filter cloth and cake are clogged, the pipe 9 is pulled up and the valve 13 of a backwashing pipe 13 is opened to feed washing liquid into the chamber 7 through the opening part thereof. The washing liquid is conducted through a communication port 8 into a filtrate chamber 3, whereby the backwashing operation is accomplished.