In a developing apparatus comprising an image support having a latent image formed on the surface, a cylindrical sleeve made of a non-magnetic metal material placed so as to face the image support, and a magnetic field-generating means placed in said sleeve, said image support and said sleeve being arranged so that a magnetic developer held on said sleeve can be conveyed to the surface of the aforesaid image support, thereby developing the aforesaid latent image, a conductive or semiconductive coating layer having a thickness of at least 1 .mu.m, a hardness of HV 900 or more and a specific resistance of 10.sup.6 .OMEGA..multidot.cm or less is formed on the surface of said sleeve. This makes the conveyance of the developer good, and enables the development to be well conducted and the wear of the sleeve to be reduced.