PURPOSE: To prevent sludge from flowing out at the operation starting time wherein the sludge-containing water flowing out conventionally occurs at the operation starting time, since sludge in raw water is precipitated and the precipitated sludge can not be floated in the case the operation is interrupted, in an apparatus in which dissolved air-containing pressured water is led to raw water in a floating tank by a jetting nozzle to float and separate the sludge in the raw water.

CONSTITUTION: An auxiliary jetting nozzle 10 is installed below a jetting nozzle 8a to inject pressured water in a tank, preferably near the bottom part and at the time operation is resumed after operation is interrupted, pressured water is led through the auxiliary nozzle before raw water is introduced into the tank, so that precipitated sludge in the tank is floated and then pressure water is led to the tank through the main nozzle 8a to carry out raw water treatment and thus operation starting is improved.