The surface of a developing roller is coated with a phenol resin layer at a higher level in triboelectric series than a toner. A positively charged toner supplied from a magnetic roller onto the developing roller is negatively charged by friction with the phenol resin layer at the higher level in the triboelectric series. As a result, the positive charge of the toner is cancelled out, and the charge amount of toner per unit weight on the developing roller (Q/M)s decreases. Applying this principle, relationship between (Q/M)s and the charge amount of toner per unit weight on the magnetic roller (Q/M)m satisfies (Q/M)s<(Q/M)m.