A developing device is provided with a developer conveying member in which a plurality of electrodes arranged on a substrate at a predetermined interval are coated with a surface protection layer, the developer conveying member being provided in a developing area that faces an image carrying body whose surface carries an electrostatic latent image, wherein developer is conveyed on the developer conveying member using a traveling-wave electric field that is formed by applying a polyphase voltage to the respective electrodes. The developing device is further provided with a supplying member for supplying the developer onto the developer conveying member, wherein (i) an effective electrode width Le of the respective electrodes in their width direction orthogonal to their arranging direction and (ii) a width Lt of a developer existing area on the supplying member, the width Lt being orthogonal to a direction of supplying the developer, are set so as to satisfy a relation of Lt