A developing device, including a casing and a seal member is provided. The casing includes a divider wall, dividing a space inside the casing into a container chamber and a developing chamber, and a developing roller. The divider wall includes an opening. The container chamber accommodates a developing agent. The seal member is configured to seal the opening. The seal member includes a ventilation section configured to allow air to pass there-through but restrict the developing agent from passing there-through at least at a part of an area that seals the opening.