A mono-component developing apparatus including a developing roller which transports toner supported on an outer circumferential face of a center portion toward a developing area by rotating while frictionally sliding on a sealing member on each of two end portions. The developing roller includes surface processing areas having a spiral shape, placed on each of outer circumferential faces of the two end portions. An angle, made by a spiral direction of each surface processing area and the rotation direction, is an acute angle. The developing roller inhibits toner leakage, with the driving torque of the developing roller being only slightly increased.