A developing roll includes a tubular sleeve driven for turn, and a magnet roll fixedly supported. An intensively magnetized block, shaped like U in cross section, is buried in the magnet roll at a location thereof where the magnet roll faces the latent electrostatic image bearing member. In this case, one side of the intensively magnetized block having a groove formed therein is directed toward the axis of the magnet roll, while the other side thereof is flush with the outer surface of the magnet roll. A material of the intensively magnetized block is more intensively magnetized, by its nature, than a material of the magnet roll. The intensively magnetized block is magnetized to form a single magnetic pole or a developing magnetic pole.