PURPOSE: To improve the rate of thikening an amino acid with a titled membrane by impregnating trialkyl phosphate and an org. liquid insoluble in water in a porous membrane.

CONSTITUTION: Trialkyl phosphate which is hardly soluble in water and has a relatively short hydrocarbon chain and an org. liquid which can form a stable liquid film in a porous membrane without swelling or dissolving considerably the porous membrane and is insoluble in water are mixed. The one surface of the porous membrane such as a commercially marketed microfilter or the like is brought into contact with the above-mentioned liquid mixture so that said liquid is impregnated in the membrane. When the liquid mixture sticking superfluously therto is lightly wiped away, the improved membrane for thickening an amino acid is obtd. as it is. If the one side of this membrane for thickening is brought into contact with a soln. contg. amino acids and an alkali metallic salt which is of about ≤4pH and provides the driving force for thickening is added thereto, the amino acids permeate the other side against the concn. gradient thereof and are thus thickened.