A developing device includes a developing vessel for storing a developer, a first conveying passage, a second conveying passage, a first conveying member, a second conveying member and a developing roller for supplying toner to a photoreceptor drum. The first conveying member has a first rotary shaft and a first conveying blade formed on the periphery of the first rotary shaft. The first conveying blade has a large-diametric ring-shaped helical blade and a small-diametric helical blade, forming a double-helical structure. Alternatively, the first conveying blade is formed of helical blades being different in phase; each helical blade is formed so that the radius varies on a cycle of the rotational angle of 180 degrees.