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 sothat 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. Disclosed by the present invention is a preferred cleaning agent containing hydrogen peroxide, a heavy metal compoundand a hydroxydicarboxylic acid, wherein the hydroxydicarboxylic acid is selected from the group consisting of malic acid, tartaric acid, tartronic acid, citramalic acid, dioxymaleic acid and dioxymalonic acid, while the heavy metal compound is an iron ion.