Printing apparatus

A color image forming apparatus, comprising:

• a conveyance mechanism including a belt moving at a certain velocity for conveying record paper in an adhered manner;
• a plurality of electrostatic recording units arranged in the direction of conveyance of the record paper for forming a latent color image in conformity with image data through optical scanning of a photosensitive drum, to develop the latent image with different color toner components to thereafter transfer the thus developed image onto the record paper on said belt;
• a positional offset detection unit including at least a sensor for optically detecting a plurality of resist marks of color toner components transferred onto said belt at predetermined pitches by said plurality of electrostatic recording units;
• said sensor including a condensing lens through which a light from a predetermined laser diode is condensed and is irradiated onto a belt transfer surface as a spot light of the order of several ten microns, one or more light receiving elements arranged at a predetermined output angle for receiving scattered light as a result of the irradiation of the spot light on the toner components transferred onto said belt transfer surface, allowing said sensor to issue a sensor detection signal;
• said positional offset detection unit detecting positions of said resist marks from said spot light detected by said sensor, at least to detect positional offsets, relative to a reference resist mark of any one color, of the resist marks of the other colors;
• and
• a positional offset correction unit for relatively correcting said plurality of recording units other than said electrostatic recording unit associated with said reference image, on the basis of said positional offset detection unit, so as to eliminate any positional offsets, relative to said reference image, of the other color images.