PURPOSE: To dilute 48% concn. sodium hydroxide solution to 25% concn. solution in a receiving tank used for receipt and dilution without depending on an integrated flowing quantity of water for dilution.

CONSTITUTION: A level of the receiving tank 20 is continuously measured at a region of 0-100% by a level detector 21, while wt. and density of a concentrated liquid and concn. to be diluted are inputted to a controlling device 22 to find a level of diluted solution at a concn. to be diluted in the receiving tank 20. And a level in the tank reaches the dilution level, an inlet valve V3 is closed and cuts off supplying water for dilution.