PURPOSE: To provide the method for detection of the defect of a reverse osmosis membrane which eliminates the contamination of the membrane surfaces, substantially prevents the degradation in a water permeation rate, is free from problems of safety of dyes and impurities and can specify a leaking point or repair the defective part of the membrane.

CONSTITUTION: This method for detection of the defect of the reverse osmosis membrane by using a dye soln. consists in using edible dyestuff which is not adsorbed on the reverse osmosis membrane as the dye and using a porous sheet-like material which adsorbs the edible dyestuff and has ion exchangeability as a means for detecting the dye leaking from the defect.