An immersion-type membrane separation apparatus (10, 120, 132) including: a treatment tank storing raw water; a plurality of membrane modules (12, 122) placed within the treatment tank for filtering the raw water stored therein, each membrane module (12, 122) having a module main body (20, 126) and a separation membrane (22, 124) as a filter retained in a given form by the module main body (20, 126); and an aerator provided below the membrane modules (12, 122) in the treatment tank, wherein the separation membrane (22, 124) of the membrane modules (12, 122) comprises a polytetrafluoroethylene porous membrane, and the separation membrane is a flat membrane; the module main body has a flat-plate configuration; and the separation membrane is affixed to the module main body in a stretched state so as to cover at least one of a front surface side and a back surface side of the module main body; an ultrasonic transducer (24, 136) is provided for cleaning the separation membrane (22, 124); a case frame that supports the plurality of membrane modules (12, 122) is fixed to an interior of the treatment tank; the plurality of membrane modules (12, 122) are arranged at given intervals in a thickness direction thereof by being supported by the case frame; the separation membrane (22, 124) of each membrane module (12, 122) is spread in a vertical direction; the module main body (20, 126) of each membrane module (12, 122) is provided with a pair of support pieces that project to widthwise opposite sides of the separation membrane (22, 124) and towards an outside; and the module main body (20, 126) is supported by the case frame at the pair of support pieces; the case frame is provided with a grasp mechanism (80, 86, 90, 96, 104) for grasping each support piece by exerting grasping force on each of the pair of support pieces in the vertical direction, and a support position adjusting mechanism (86, 90) for adjusting a grasp position of the pair of support pieces by the grasp mechanism (80, 86, 90, 96, 104) with respect to the case frame in the vertical direction; and the grasp mechanism (80, 86, 90, 96, 104) includes a pair of longitudinal connecting members (80, 96) extending in an arranging direction of the plurality of membrane modules (12, 122) while being superposed against the support pieces of the respective membrane modules (12, 122) from vertically opposite sides, and a fastening member (86, 90, 104) for fastening the pair of the connecting members (80, 96) in a direction of getting closer to each other so as to grasp the support pieces of the respective membrane modules (12, 122) between the pair of the connecting members (80, 96). the separation membrane (22, 124) of the membrane modules (12, 122) comprises a polytetrafluoroethylene porous membrane, and the separation membrane is a flat membrane; the module main body has a flat-plate configuration; and the separation membrane is affixed to the module main body in a stretched state so as to cover at least one of a front surface side and a back surface side of the module main body; an ultrasonic transducer (24, 136) is provided for cleaning the separation membrane (22, 124); a case frame that supports the plurality of membrane modules (12, 122) is fixed to an interior of the treatment tank; the plurality of membrane modules (12, 122) are arranged at given intervals in a thickness direction thereof by being supported by the case frame; the separation membrane (22, 124) of each membrane module (12, 122) is spread in a vertical direction; the module main body (20, 126) of each membrane module (12, 122) is provided with a pair of support pieces that project to widthwise opposite sides of the separation membrane (22, 124) and towards an outside; and the module main body (20, 126) is supported by the case frame at the pair of support pieces; the case frame is provided with a grasp mechanism (80, 86, 90, 96, 104) for grasping each support piece by exerting grasping force on each of the pair of support pieces in the vertical direction, and a support position adjusting mechanism (86, 90) for adjusting a grasp position of the pair of support pieces by the grasp mechanism (80, 86, 90, 96, 104) with respect to the case frame in the vertical direction; and