Abstract: The subject invention provides a developing device in which respective lateral ends (the portions overlapped with the side seals 27) of the front seal board 23 are adhered/fixed to the side seals 27. More specifically, the gap between the both sides A1 and the return edge A2 of the opening section A of the toner tank 49 and the developing roller is sealed by a single plane composed of the side seals 27 and the front seal board 23 that are combined with each other. On this account, there is no gap between the side seal 27 and the front seal board 23 due to vibration caused by external force, time degradation etc. (i.e., it prevents the side seals 27 to be away from the front seal board 23). On this account, toner leakage can be securely prevented even when the developing device is driven for a long period of time or when some kind of vibration is applied to the development device.