A submerged membrane separator 31 includes: a casing 33 opened at the top; a plurality of flat membrane cartridges 34 arrayed at intervals in the casing 33; and an air diffuser 36 provided below the membrane cartridges 34. The membrane cartridges 34 are movably held in the up-down direction in the casing 33 while having a space α in the up-down direction. Lifting suppressing members 63a and 63b for restricting a lifting amount of the membrane cartridges 34 during filtration operation to be smaller than the space α in the up-down direction are provided in the casing 33.