PURPOSE: To enable to cool sludge accumulated in a rotary container even when the rotation of the rotary container happens to discontinue and control the discharge of harmful substances following the overheat of sludge.

CONSTITUTION: In a solid-liquid separator 31 consisting of a cylindrical rotary container 31 for centrifugal separating introduced solution into a slid content and supernatant liquid and a driving mechanism 6 for rotation driving said rotary container 31, a cooling water introducing pipe 40 is provided in the rotary container 31 and cooling water is discharged to the whole of the inner face of the rotary container 31 to cool the solid content.