A developing unit including: a first agitating and conveying rotor and second agitating and conveying rotor for agitating and conveying toner inside a casing; and a developing roller for supplying the toner to a photoreceptor drum is constructed such that the first agitating and conveying rotor has a rotary shaft arranged approximately parallel to the developing roller and a multiple number of agitating vanes arranged on the rotary shaft with an inclination to the axial direction of the rotary shaft; each agitating element is formed asymmetrically about a normal that passes through the rotational center and adjacent agitating elements are asymmetrical to each other; and an odd number of asymmetrical agitating elements are arranged alternately.