An image forming apparatus comprising: a main casing; a plurality of image forming units juxtaposedly arrayed with each other in a predetermined direction, each of the plurality of image forming units comprising a photosensitive drum on which an electrostatic latent image is formed, a developer supporting member disposed in confrontation with the photosensitive drum, and a reception opening configured to receive developer, each of the plurality of photosensitive drums having an axis extending in an axial direction perpendicular to the predetermined direction; a plurality of developer cartridges juxtaposedly arrayed with each other in the predetermined direction and each provided for each of the plurality of image forming units, each of the plurality of developer cartridges disposed in confrontation with corresponding one of the plurality of image forming units in the axial direction, each of the plurality of developer cartridges comprising a developer accommodating section configured to accommodate developer and a supply opening (97, 99) through which developer in the developer accommodating section is supplied to corresponding one of the plurality of image forming units; and a cartridge supporting unit detachably supporting each of the plurality of developer cartridges; characterized by a moving mechanism configured to move the cartridge supporting unit to a mounted position, a retracted position, and a pulled-out position, wherein the cartridge supporting unit is mounted in the main casing and the supply opening and the reception opening are in communication with each other when the cartridge supporting unit is in the mounted position, the cartridge supporting unit is moved away from the plurality of image forming units in the axial direction to interrupt a communication between the supply opening and the reception opening when the cartridge supporting unit is in the retracted position, and the cartridge supporting unit is pulled outside the main casing from the retracted position in the predetermined direction when the cartridge supporting unit is in the pulled-out position. a main casing; a plurality of image forming units juxtaposedly arrayed with each other in a predetermined direction, each of the plurality of image forming units comprising a photosensitive drum on which an electrostatic latent image is formed, a developer supporting member disposed in confrontation with the photosensitive drum, and a reception opening configured to receive developer, each of the plurality of photosensitive drums having an axis extending in an axial direction perpendicular to the predetermined direction; a plurality of developer cartridges juxtaposedly arrayed with each other in the predetermined direction and each provided for each of the plurality of image forming units, each of the plurality of developer cartridges disposed in confrontation with corresponding one of the plurality of image forming units in the axial direction, each of the plurality of developer cartridges comprising a developer accommodating section configured to accommodate developer and a supply opening (97, 99) through which developer in the developer accommodating section is supplied to corresponding one of the plurality of image forming units; and a cartridge supporting unit detachably supporting each of the plurality of developer cartridges; characterized by a moving mechanism configured to move the cartridge supporting unit to a mounted position, a retracted position, and a pulled-out position, wherein the cartridge supporting unit is mounted in the main casing and the supply opening and the reception opening are in communication with each other when the cartridge supporting unit is in the mounted position, the cartridge supporting unit is moved away from the plurality of image forming units in the axial direction to interrupt a communication between the supply opening and the reception opening when the cartridge supporting unit is in the retracted position, and the cartridge supporting unit is pulled outside the main casing from the retracted position in the predetermined direction when the cartridge supporting unit is in the pulled-out position.