PROBLEM TO BE SOLVED: To provide a method for filtering a biological fluid, where both removing white blood cells with high efficiency and shortening the filtration time are possible.SOLUTION: The method is for filtering a biological fluid, which includes (a) the step where volume% or above 50 and 80 volume% or below of the total fluid is filtered at a linear velocity of 0.20 to 1.50 ml/m/cmand (b) the linear velocity at the step (a) is changed into 0.04 to 0.14 ml/m/cmlower than the step (a), and the remaining biological fluid is filtered with such a low linear velocity these steps are based on the assumptions that the total number of white blood cells contained in all the biological fluid to pass a filter is N, that the filtration area of the filter is A cm, and that the ratio N/A is 2.8×10/cmto 2.3×10/cm.
