DEVELOPING DEVICE

PURPOSE:To prevent the blowoff of toner which rides the air blown off from a vent part toward the outside by providing the vent part to the outside at the part of a developer storing area and applying a semipermeable film to the vent part. CONSTITUTION:The semipermeable film 85 is applied to the vent part provided at the part of the developer storing area of a developing device. Therefore, it transmits only air and does not transmits toner. By selecting the total aperture area of the event part, the resistance thereof is made much smaller than ventilation resistance through a seal part 25 around the developing roller 11 of the developing device. Thus, the developing device is attached/detached or replaced singly or in cartridge unit with others. In the case that inside air is blown off because of deformation that content volume becomes small in handling when the device is carried, the toner riding the air blown off from the vent part is captured by the film 85 and the blowoff of the toner toward the outside is prevented.