An image forming apparatus includes an image carrier, a developing unit, a concentration sensor, a toner supplying unit, a developer supplying unit, an ejector, and a condition detector. The image carrier forms a latent image thereon. The developing unit develops the latent image formed on the image carrier with a two-component image developer including toner particles and carrier particles. The concentration sensor detects a toner ratio in the developing unit. The toner supplying unit supplies fresh toner particles to the developing unit. The developer supplying unit supplies fresh image developer to the developing unit. The ejector ejects the image developer to an outside of the developing unit. The condition detector detects a condition of the image developer used for an image forming operation to determine a supply amount of the image developer to supply to the developing unit. The condition includes a degradation level of the image developer.