An image forming apparatus of the present invention disclosed is of the type sequentially transferring toner images from a plurality of photoconductive drums to a sheet being conveyed by an image transfer belt or an intermediate image transfer belt one above the other with bias applying members to thereby form a composite color image. Backup rollers, contacting the inside surface of the belt, each have volumetric resistivity of 10.sup.9 .OMEGA.cm or above and ten-point mean surface roughness Rz of 6 .mu.m or above.