According to one embodiment, a developing device includes a developer container to contain developer having toner and carrier, a toner replenishing unit to replenish the toner to the developer container, a carrier replenishing unit to replenish the carrier to the developer container, a discharge portion to discharge the developer from the developer container, a developer feeder to support magnetically the developer in the developer container and to feed the toner in the developer to a photoconductor on which an electrostatic latent image is formed, an electric conductor, provided separately from the photoconductor, to which the toner out of the developer supported by the developer feeder adheres, a detecting unit to detect a density of the toner adhered to on the electric conductor, and a controller to judge a deterioration status of the developer based on a detection result of the detecting unit and to replenish the toner and the carrier from the toner replenishing unit and the carrier replenishing unit to the developer container.