PURPOSE: To reduce the amount of work and to attain a real-time analysis of constituents by executing a series of operations of sampling, filtration, conveyance of samples and washing of a filter automatically and continuously.

CONSTITUTION: With valves (a), (e), (j) and (m) opened, a suction device 6 is started and a slurry liquid in a tank is filled up in a precipitation vessel 2 through a liquid contact pipe 1. Then, the valves (a), (e), (j) and (m) are closed and the device 6 is stopped. Next, the vessel 2 is opened to the air by opening a valve (f) and slurry in the liquid is subjected to spontaneous precipitation sufficiently. Thereafter the device 6 is started with valves (b), (j) and (m) opened while the valve (f) left open, and a supernatant liquid in the vessel 2 is made to flow into a sample vessel 5 through a filter 4 via a pipeline 11. At the time point when a prescribed quantity of this liquid is stored, the valves (b), (j) and (m) are closed and the device 6 is stopped. After sampling operations stated above are finished, the filtered liquid in the vessel 5 is pressurized by compressed air by opening valves (l) and (o) and sent under the pressure to a supply device 8 via a pipeline 14. Opening a valve (q) as occasion demands, the device 8 supplies the filtered liquid to an analyzing device.