Abstract: The present invention relates to a method for diluting an acid or alkaline stock solution with pure water to prepare a mixed solution of an aimed concentration, which method comprises:an initial preparation step A, in which the prescribed amounts of a stock solution and pure water are mixed to prepare a mixed solution with a concentration different from the preset target concentration;a concentration measuring step B, in which the concentration of the mixed solution is measured by the potentiometric titration method; anda preparation step C, in which the deficiency of the stock solution or pure water is calculated on the basis of the difference between the value of concentration measured in the step B and the value of target concentration, and 70 to 99% of the calculated deficiency of the stock solution or pure water is supplied to the mixed solution,the said steps B and C being repeated until the measured value of concentration falls within the range of the threshold of the preset target concentration.