A developing unit has a developer housing and a developing roll contained therein, the developer housing holding a two-component developer composed of carrier and toner. The developing roll is placed facing the opening of the developer housing and provided with a magnet member and a developing sleeve. The magnet member has multiple magnetic poles fixed thereto, and the developing sleeve is rotatably slipped on the magnet member. The developing sleeve is formed from an aluminum substrate by roughening its surface and subsequently etching the roughened surface so that the resulting etched part has an average slope angle of 15 to 20.degree..