Abstract: In a method for cleaning a membrane module including at least one of a microfiltration membrane and an ultrafiltration membrane through which raw water is membrane-filtered to obtain membrane filtrate, a chemical diffusion step is provided in which chemical-containing water is fed to a feed side of the membrane module and diffused from the feed side of the membrane module to a filtrated side of the membrane module is performed, and subsequently a backwashing step is provided in which the membrane module is backwashed by allowing the membrane filtrate to flow from the filtrated side to the feed side of the membrane module is performed, in which, in the chemical diffusion step, an execution time of the chemical diffusion step is controlled based on a concentration of the chemical diffused to the filtrated side of the membrane module.