A detergent and a method for washing applicable to selectively permeable membranes are proposed, which can attain washing of selectively permeable membranes deteriorated in its membrane performance, such as the flux, in an efficient manner within a brief time at a higher detergent effect without causing deterioration of the selectively permeable membranes and without damaging safety to human and to environment with superior handling aspect. A detergent comprising a polyol having a molecular weight not higher than 400 and, if necessary, an organic solvent is supplied to the concentrate chamber 3 of the module 1 to carry out washing of the selectively permeable membrane 2 by bringing it into contact with the selectively permeable membrane 2 and, on requirement, water or an alkaline solution is further supplied from the water tank 6 or from the alkali storage 8 to the concentrate chamber 3 of the module 1, in order to recover the membrane performance of the selectively permeable membrane 2.