According to one embodiment, an image forming apparatus includes a developing unit configured to store a developer includes a toner and a carrier and to form a developer image, a toner attachment amount detection portion configured to detect an attachment amount of a toner formed on a photoconductor or an intermediate transfer body, a developing bias voltage change portion configured to change a plurality of developing bias voltages including a developing contrast voltage in a manner that the detected attachment amount of the toner becomes a value in a predetermined range, a carrier supply time correction portion configured to correct a carrier supply time in response to the value of the changed developing contrast voltage, and a carrier supply portion configured to supply a carrier in a corrected carrier supply time at a predetermined timing.