To provide a rotary drum filter cloth type solid/liquid separator that can obtain a clear separated liquid at a stabilized treatment capacity with a small driving power and to provide a method for treating sludge or sewage using the separator.

This separator is a rotary drum filter cloth type solid/liquid separator used for concentrating a suspended substance and having a double structure consisting of a rotary drum 1 over which a filter cloth 2 is put up and an outer drum 3, wherein the rotary drum 1 has an inlet 5 for the material to be treated at an upstream of its inside and the drum 3 has an exit 6 for the separated liquid. The filter cloth 2 is suitably a synthetic resin woven cloth having an air permeability of 10,000-1,200 cm3/cm2.min, and a feed flight 4 may be provided in the inside of the filter cloth. This method is one wherein the above separator is used for the solid/liquid separation of sludge or sewage to obtain concentrated sludge and a clear treated liquid.