A multicolor image forming apparatus includes a plurality of developing units and a development power supply unit. Each of the developing units includes a developing roller located at a development gap from a photosensitive body, deposits toner accommodated therein on a surface of the developing roller, and supplies the toner to the photosensitive body across the development gap. The development power supply unit applies a development bias voltage to developing rollers of the developing units, the developing bias voltage being a rectangular AC bias voltage in which a forward bias voltage and a reverse bias voltage alternate. A percentage of toner particles of the toner having diameter-charge amounts greater than a contamination limit diameter-charge amount of a toner particle is less than 5%.