Even with an extend use of a laser printer, toner leakage can be prevented. A side seal disposed at each end of a developing roller includes a sealing member that makes sliding contact with a surface of the developing roller and a sponge seal that supports the sealing member. The sponge seal includes a base portion and a projecting portion. The base portion has a substantially rectangular shape, to which the sealing member is affixed. The projecting portion projects from the center of the base portion toward the inside of the developing roller along its length, without being caught between a supply roller and the developing roller.