Abstract: A desalination performance restoration agent for a cellulose acetate membrane contains: a solvent; and a modified polyvinyl alcohol mixed in the solvent. The modified polyvinyl alcohol has an acetyl group structure in at least part of polyvinyl alcohol.

Abstract: Provided is a cleaning agent or cleaning method of a separation membrane which has cleaning effects equal or superior to those of conventional cleaning methods in cleaning a separation membrane used for filtration of drinking water, industrial water, river water, lake water, groundwater, reservoir water, secondary effluent, waste water, sewage water or the like; does not include a hypochlorite so that it neither generates a toxic gas nor adversely affects the human health; and needs less cleaning time owing to elimination of a rinsing step with a large amount of water. The cleaning agent or cleaning method according to the present invention is therefore safe and economical.