A developing device including a developer supporter, and a napping unit for napping the developer on the developer supporter by magnetic poles. The napping unit further includes a developing main magnetic pole installed at a location opposite to a latent image supporter, and a developing auxiliary magnetic pole located at a downstream side of the developing main magnetic pole in a rotational direction of the developer supporter. The developing main magnetic pole has a half-value width equal to or less than 22°. The composite magnetic flux density of the developing main magnetic pole and the developing auxiliary magnetic pole is equal to or greater than 80 mT. In this way, the disclosure can prevent voids in a back end from occurring to improve the fine line reproduction, and additionally, the carrier adhesion can be also prevented.