To provide a method for thickening and dehydrating sludge capable of dehydrating hardly-dehydratable sludge without adding much coagulant thereto, and an apparatus for thickening the sludge used therefor.

Coagulated sludge is thickened through a thickening apparatus 3 by separating from water without breaking floe and is dehydrated by force feeding to a pressure dehydrator. The thickening apparatus 3 has a structure wherein a screen 11 is provided in a pressure vessel 10, inside of the screen being a sludge channel 12 and outside being a filtrate chamber 13, and the sludge channel 12 is provided therein with a hollow scraper 18, slowly rotating along the inner surface of the screen 11, the hollow scraper 18 having through holes, through which the filtrate in the filtrate chamber 13 is sucked in via the screen 11.