This membrane filtration method includes: a membrane filtration process for adding a coagulant to water to be treated which contains viruses, and filtering the water to be treated by using a filtration membrane; and a cleaning process for, after the membrane filtration process, cleaning the filtration membrane, the membrane filtration process and the cleaning process being repeatedly performed, wherein, in the membrane filtration process, in the initial period of filtration, at least either an operation of filtering the water to be treated that includes the coagulant added by an amount larger than the amount of a coagulant added in a normal case, or an operation of filtering the water to be treated that has a pH lower than the pH of water to be treated in a normal case, is performed.