An apparatus for dewatering primary dewatered sludge pieces conditioned with dewatering powders comprises: a first member having a recess and a path communicating with the recess, a fluid-tight diaphragm mounted over the recess thereby defining a fluid chamber with which the path communicates, a second member having a filtrate member, and a filter member provided over the filtrate chamber. At least one of the first member and the second member is mounted to bring them into fluid-tight contact with each other, and the primary dewatered sludge pieces into a cavity defined by the diaphragm and the filter member. A pressure fluid then is fed to the fluid chamber, thereby isostatically compressing the primary dewatered sludge pieces to remove the water contained therein without substantially destroying the inner structures of the primary dewatered sludge pieces. Because of this isostatic pressure exerted by the diaphragm, the sludge pieces are compressed highly uniformly, resulting in high dewatering efficiency.