PURPOSE: To improve the treating efficiency of the titled appratus by giving diagonal reciprocative motion to a pair of filters disposed in a treating chamber with a sludge passage in-between and periodically deforming the frame set and filter elements to a parallelogram shape.

CONSTITUTION: The sludge to be dehydrated is continuously supplied to a treating chamber 12 through a sludge feed port 16. When the sludge is dehydrated in filters 1 and given cake is formed on the surfaces of the filters 1, the filters 1 are pulled diagonally by drive shafts 10. Then, the planes of the two filters 1 become roughly parallel. Next, when they are pushed diagonally by the shafts 10, the upper parts of the opposing two filters 1 expand and the new sludge is filled on the filter 1 surfaces. The dehydrated sludge moves downward. By repeating the above-mentioned operation, the dehydrated solids are discharged to outside the chamber 12 through a discharge port 13 and the separated water is discharged to outside the chamber 12 through a water feed port 11.