1. A developing device, comprising: a transporting member arranged opposite to a latent image supporter and configured to develop a latent image on the latent image supporter with a powder while moving the powder; said transporting member comprising a plurality of electrodes configured to generate a traveling-wave electric field to move the powder, wherein an average potential of the n-phase voltages applied to the plurality of electrodes of the transporting member is set to a potential between a potential of the image portion of the latent image and a potential of the non-image portion of the latent image.