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 carrying 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 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 member having a supply opening through which developer in the developer accommodating section is supplied to corresponding one of the plurality of image forming units; and a support member detachably supporting each of the plurality of developer cartridges, characterized in that the support member is configured to be moved in the predetermined direction between a mounted position in which the support member is mounted in the main casing and a pulled-out position in which the support member is pulled out of the main casing; and by a moving mechanism configured to move the supply member relative to the developer cartridge between a communication position and an interrupt position when the support member is in the mounted position, wherein, when the supply member is in the communication position, the supply opening and the reception opening are in communication with each other and the support member is incapable of moving in the predetermined direction, and, when the supply member is in the interrupt position, the supply member is moved away from the corresponding one of the plurality of image forming units in the axial direction to interrupt communication between the supply opening and the reception opening and the support member is movable toward the pulled-out position in the predetermined direction. 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 carrying 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 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 member having a supply opening through which developer in the developer accommodating section is supplied to corresponding one of the plurality of image forming units; and a support member detachably supporting each of the plurality of developer cartridges, characterized in that the support member is configured to be moved in the predetermined direction between a mounted position in which the support member is mounted in the main casing and a pulled-out position in which the support member is pulled out of the main casing; and by a moving mechanism configured to move the supply member relative to the developer cartridge between a communication position and an interrupt position when the support member is in the mounted position, wherein, when the supply member is in the communication position, the supply opening and the reception opening are in communication with each other and the support member is incapable of moving in the predetermined direction, and, when the supply member is in the interrupt position, the supply member is moved away from the corresponding one of the plurality of image forming units in the axial direction to interrupt communication between the supply opening and the reception opening and the support member is movable toward the pulled-out position in the predetermined direction.