developing roller driving motor different from the common developing roller driving motor for driving the developing rollers 34 y 34 m and 34 c the chargers 2 corresponding to the y m c and k colors are connected to an unillustrated common charging power supply and a charging bias is applied thereto the developing rollers 34 y 34 m and 34 c corresponding to the y m and c colors are connected to an unillustrated common developing power supply and a developing bias is applied thereto the developing roller 34 k is connected to a developing power supply different from the common developing power supply connected to the developing rollers 34 y 34 m and 34 c and a developing bias is applied thereto the photoconductor driving motor and the charging power supply are common to the image forming sections 9 corresponding to the y m c and k colors and the developing roller driving motor and the developing power supply are common to the image forming sections 9 y 9 m and 9 c corresponding to the y m and c colors in order to reduce the number of components and the total cost of the image forming apparatus 10.