An image forming apparatus includes a plurality of removable developing units each of which has an image carrier whose width is narrower than a width of a printable area of the printing paper, and a driving unit for driving the plurality of the removable developing units. The plurality of the removable developing units are disposed in different rows parallel to a main scanning direction such that one end of an image producing area of an image carrier of a first developing unit coincides with one end of an image producing area of an image carrier of a second developing unit adjoining to the first developing unit. The image forming apparatus further includes a control unit for supplying printing data to the plurality of the removable developing units in timings shifted form row to row.