A developing device includes a container for containing a magnetic developer therein, a developer carrier for carrying and conveying the magnetic developer in the container, a magnetic member for forming a magnetic seal area of a predetermined gap with respect to the surface of the developer carrier in the end portions of the developer carrier along the circumferential direction of the developer carrier. The magnetic member has a plurality of magnetic poles along the circumferential direction of the developer carrier in the magnetic seal area, and an external magnetic pole provided outside the endmost magnetic pole forming the magnetic seal for conveying a magnetic flux travelling from the endmost magnetic pole toward the outside of the magnetic seal area.