A submerged membrane separator comprising: a casing opened at a top; a plurality of flat membrane cartridges arrayed at intervals in the casing; an air diffuser provided below the membrane cartridges; wherein the membrane cartridges are movably held in an up-down direction in the casing while having a space (α) in the up-down direction, a lifting suppressing member (63a, 63b) for restricting a lifting amount of the membrane cartridges during a filtration operation to be smaller than the space (α) in the up-down direction is provided in the casing; the separator is arranged such that during the filtration operation, an upflow is generated among the membrane cartridges by bubbles of air diffused from the air diffuser and an upward force acts on the membrane cartridges due to the upflow and the membrane cartridges are liftable with the upward force where the lifting amount of the membrane cartridges is restricted by the lifting suppressing member (63a, 63b) to be smaller than the space (α) in the up-down direction; characterized in that: a portion of the lifting suppressing member (63a, 63b) that comes into contact with the membrane cartridges is an elastic member and upper guiding members and lower guiding members for guiding the membrane cartridges in the width direction are provided in the inside of the casing, the left and right upper guiding members are provided in pairs in an upper frame of a frame body and the left and right lower guiding members are provided in pairs in the bottom frame; a plurality of front and rear guiding grooves are formed in each of the upper and lower guiding members (70, 71) where the upper ends of the membrane cartridges are freely inserted into and removed from the guiding grooves of the upper guiding member from the width direction and the lower ends of the membrane cartridges are freely inserted into and removed from the guiding grooves of the lower guiding member from the width direction; where the height H1 between an upper inner surface of the guiding groove of the upper guiding member and a lower inner surface of the guiding groove of the lower guiding member is set larger than a height H2 of the membrane cartridge, such that the space (α) in the up-down direction is defined as α = H1-H2. a casing opened at a top; a plurality of flat membrane cartridges arrayed at intervals in the casing; an air diffuser provided below the membrane cartridges; wherein the membrane cartridges are movably held in an up-down direction in the casing while having a space (α) in the up-down direction, a lifting suppressing member (63a, 63b) for restricting a lifting amount of the membrane cartridges during a filtration operation to be smaller than the space (α) in the up-down direction is provided in the casing; the separator is arranged such that during the filtration operation, an upflow is generated among the membrane cartridges by bubbles of air diffused from the air diffuser and an upward force acts on the membrane cartridges due to the upflow and the membrane cartridges are liftable with the upward force where the lifting amount of the membrane cartridges is restricted by the lifting suppressing member (63a, 63b) to be smaller than the space (α) in the up-down direction; characterized in that: a portion of the lifting suppressing member (63a, 63b) that comes into contact with the membrane cartridges is an elastic member and upper guiding members and lower guiding members for guiding the membrane cartridges in the width direction are provided in the inside of the casing, the left and right upper guiding members are provided in pairs in an upper frame of a frame body and the left and right lower guiding members are provided in pairs in the bottom frame; a portion of the lifting suppressing member (63a, 63b) that comes into contact with the membrane cartridges is an elastic member and upper guiding members and lower guiding members for guiding the membrane cartridges in the width direction are provided in the inside of the casing, the left and right upper guiding members are provided in pairs in an upper frame of a frame body and the left and right lower guiding members are provided in pairs in the bottom frame; a plurality of front and rear guiding grooves are formed in each of the upper and lower guiding members (70, 71) where the upper ends of the membrane cartridges are freely inserted into and removed from the guiding grooves of the upper guiding member from the width direction and the lower ends of the membrane cartridges are freely inserted into and removed from the guiding grooves of the lower guiding member from the width direction; where the height H1 between an upper inner surface of the guiding groove of the upper guiding member and a lower inner surface of the guiding groove of the lower guiding member is set larger than a height H2 of the membrane cartridge, such that the space (α) in the up-down direction is defined as α = H1-H2.