An electrode module in the form of a flexible sheet which contacts with a toner layer and to which a bias voltage is applied is provided on a developing roller before development. The electrode module is adapted to separate the toner layer largely into a toner-rich layer and a carrier-rich layer by utilizing electrical migration occurring in the toner layer when an electric field is applied to the toner layer on the basis of the bias voltage. The electrode module is constructed in such a manner that the sheet is pulled toward a surface of the developer bearer body by a force attributable to surface tension or wetting characteristic of the liquid developer existing between the sheet and the developer carrying body, whereby the sheet comes into contact with the surface of the developer bearer body. This electrode construction increases the contact area or nip width of a bias blade provided for the developing roller and enhances its contact stability.