To improve the mobility of a purifying ship and the purification ratio of river system by providing a lower layer water injecting device for injecting a gas pressurized water delivered from an air dissolving device to the stem side water area within a water passage space.

The gas pressurized water delivered from a gas dissolving device 68 and injected through a distributing device 87 to a floating separating device 55 is injected to a first water filling pipe and/or a second water filling pipe according to the property and quantity of a water A to be treated such as sludge water, reformed sludge water, coagulated sludge water, coagulated reformed sludge water or the like delivered from a feed switching device 52. The gas pressurized water injected to the first water filling pipe is supplied from a dispersion nozzle while being mixed into to the water A to be treated carried in an injection pipe 53, and the gas pressurized water injected into the second water filling pipe is supplied from the dispersion nozzle to the lower layer water area of a floating tank 55a.