A developing device including a developing roller mounted in a developing chamber is provided. In the developing device, the developing roller is partially exposed to an environment outside the developing chamber through an opening portion of the developing chamber, an air inlet is provided between an upstream boundary of the opening portion in a rotational direction of the developing roller and an outercircumference of the developing roller, wherein air may pass into the developing chamber through the air inlet, and an air outlet may be formed near the upstream boundary of the opening portion and communicatively connected to the developing chamber so as to discharge the air in the developing chamber. The air outlet is located outside an effective image area in a length direction of the developing roller.