To effectively use a filter membrane without causing problems by the recovery and processing a washing water, by controlling a waste water from a washing tank to specified pH, treating the whole waste water in a single filtering device, controlling the hardness of water of the transmitted liquid to a specified value, and recycling the processing water to one tank in the washing zone.

The waste water included in the washing zone of processing process lines 10 to 13 is recovered and sent through a buffer tank 15 to a filtering unit 14 which is common to the all processing processes 10 to 13. A tank 16 contains a basic or acidic soln. to control the waste water in the tank 15 to 6 to 8 pH range, preferably 6.5 to 7.5. Acetic acid is used to decrease the pH of the waste water, while an aq. soln. of sodium hydroxide is used to increase the pH. The transmitted liquid produced in the filtering unit 14 is sent to a reservoir 19 where a salt water is supplied from a tank 20 so as to add Ca by 5 mg per 1 l of the transmitted liquid, and the water hardness of the transmitted liquid is adjusted to >1. The transmitted liquid flowing out of the reservoir 19 is sent to one of the washing zone 10 to 13 in the processing process.