PURPOSE: To completely wash the inside of a decanter by boring a drain hole in the outermost diameter part of the end plate of a rotary bowl and forming a flow path of washing liquid between an intubation hole and a raw liquid feeding pipe and providing a washing liquid extracting chamber communicated with the flow path of washing liquid to the end part of the raw liquid feeding pipe.

CONSTITUTION: A decanter is constituted of both a screw conveyor 5 which is rotated uniaxiably with a rotary bowl 4 at a little differential velocity and a raw liquid feed pipe 6 inserted through an intubation hole 9 of the end plate 8 of the rotary bowl 4. Further a drain hole 10 is bored to the outermost diameter part of the end plate 8 and a flow path 13 of washing liquid is formed between the intubation hole 9 and the feed pipe 6. A washing liquid extracting chamber 11 communicated with the flow path 13 is provided to the end part of the feed pipe 6. Both a discharge port 2 of solid material and a discharge port 3 of separated liquid are plugged. Washing liquid is supplied through the feed pipe 6 and extracted from the extracting chamber 11 while rotating the bowl 4 and the conveyor 5. As a result, the inside of the decanter is completely washed.