1. A method of transferring a plurality of toner images formed on a plurality of image carriers of different colors onto a sheet-type recording medium, the method comprising: forming a mark image on each of the image carriers as a toner image; transferring each of the mark images to a common transfer medium; detecting optically a positional relationship of the collective mark images relative to one another; and controlling a linear velocity of each of the image carriers based on a result of detecting the positional relationship of the collective mark images relative to one another, to transfer the toner images onto the sheet-type recording medium with a reduced misalignment between the toner images.