A transverse-mounted membrane filtration apparatus comprising: a membrane module comprising a casing having a raw water inlet, a filtrate water outlet and a concentrated water outlet, and a membrane element loaded in the casing; an inlet pipe through which a raw water flows to the membrane module, the raw water flowing in a flow path of a membrane element of the membrane module in an approximately horizontal direction; and an air diffuser device including a shelf-shaped member and an air-feeding unit, the shelf-shaped member including a plurality of shelf boards (11a-i) being arranged in a flow path segment at an immediate upstream of an end surface of the membrane element with a predetermined distance from the end surface of the membrane element and covering the entire flow path of the membrane element along a vertical direction, characterized in that: the air-feeding unit feeds air to the raw water for washing the membrane element through an air-feeding port provided at a bottom portion of a flow path of the shelf-shaped member, wherein a shelf board at a lowermost position of the shelf-shaped member is inclined toward a side of the end surface of the membrane element in a vertically upper direction so that the shelf board at the lowermost position of the shelf-shaped member along a vertical direction disperses and divides the air blown out from the air-feeding port into a first air portion to be forced into a flow path of a lower region of the end surface of the membrane element by a predetermined ratio through a gap between the shelf-shaped member and the end surface of the membrane element and a second air portion to be detoured from an upstream side of the shelf-shaped member and forced into a flow path of an upper region of a rest of the end surface of the membrane element. a membrane module comprising a casing having a raw water inlet, a filtrate water outlet and a concentrated water outlet, and a membrane element loaded in the casing; an inlet pipe through which a raw water flows to the membrane module, the raw water flowing in a flow path of a membrane element of the membrane module in an approximately horizontal direction; and an air diffuser device including a shelf-shaped member and an air-feeding unit, the shelf-shaped member including a plurality of shelf boards (11a-i) being arranged in a flow path segment at an immediate upstream of an end surface of the membrane element with a predetermined distance from the end surface of the membrane element and covering the entire flow path of the membrane element along a vertical direction, characterized in that: the air-feeding unit feeds air to the raw water for washing the membrane element through an air-feeding port provided at a bottom portion of a flow path of the shelf-shaped member, wherein a shelf board at a lowermost position of the shelf-shaped member is inclined toward a side of the end surface of the membrane element in a vertically upper direction so that the shelf board at the lowermost position of the shelf-shaped member along a vertical direction disperses and divides the air blown out from the air-feeding port into a first air portion to be forced into a flow path of a lower region of the end surface of the membrane element by a predetermined ratio through a gap between the shelf-shaped member and the end surface of the membrane element and a second air portion to be detoured from an upstream side of the shelf-shaped member and forced into a flow path of an upper region of a rest of the end surface of the membrane element. the air-feeding unit feeds air to the raw water for washing the membrane element through an air-feeding port provided at a bottom portion of a flow path of the shelf-shaped member, wherein a shelf board at a lowermost position of the shelf-shaped member is inclined toward a side of the end surface of the membrane element in a vertically upper direction so that the shelf board at the lowermost position of the shelf-shaped member along a vertical direction disperses and divides the air blown out from the air-feeding port into a first air portion to be forced into a flow path of a lower region of the end surface of the membrane element by a predetermined ratio through a gap between the shelf-shaped member and the end surface of the membrane element and a second air portion to be detoured from an upstream side of the shelf-shaped member and forced into a flow path of an upper region of a rest of the end surface of the membrane element.