An image forming apparatus of a single pass scheme, comprising: a photosensitive drum having an outer circumferential surface on which an electrostatic latent image is formed; an exposure unit configured to radiate light at the photosensitive drum to form the electrostatic latent image on the outer circumferential surface of the photosensitive drum ; a developing unit configured to form a toner image by supplying a color toner that corresponds to the electrostatic latent image formed on the outer circumferential surface of the photosensitive drum ; an intermediate transfer belt to which the toner image formed at the outer circumferential surface of the photosensitive drum is transferred; a pattern generating unit configured to form an electrostatic latent image corresponding to a predetermined Auto Color Registration 'ACR' pattern on the outer circumferential surface of the photosensitive drum to form the ACR pattern on the intermediate transfer belt ; and a pattern detecting unit configured to detect the ACR pattern that is formed on the intermediate transfer belt ; characterised in that: the pattern generating unit is configured to allow amounts of gap changes of a plurality of sub patterns, which together form the ACR pattern, to have an average value of about 0 such that a DC offset can be accurately determined, wherein the gap change of the plurality of sub patterns is caused by an AC component generated from a rotation of the photosensitive drum ; and an ACR executing unit is configured to calculate an offset of each color based on the detection result of the pattern detecting unit , and to correct a color registration error by use of the offset calculated. a photosensitive drum having an outer circumferential surface on which an electrostatic latent image is formed; an exposure unit configured to radiate light at the photosensitive drum to form the electrostatic latent image on the outer circumferential surface of the photosensitive drum ; a developing unit configured to form a toner image by supplying a color toner that corresponds to the electrostatic latent image formed on the outer circumferential surface of the photosensitive drum ; an intermediate transfer belt to which the toner image formed at the outer circumferential surface of the photosensitive drum is transferred; a pattern generating unit configured to form an electrostatic latent image corresponding to a predetermined Auto Color Registration 'ACR' pattern on the outer circumferential surface of the photosensitive drum to form the ACR pattern on the intermediate transfer belt ; and a pattern detecting unit configured to detect the ACR pattern that is formed on the intermediate transfer belt ; characterised in that: the pattern generating unit is configured to allow amounts of gap changes of a plurality of sub patterns, which together form the ACR pattern, to have an average value of about 0 such that a DC offset can be accurately determined, wherein the gap change of the plurality of sub patterns is caused by an AC component generated from a rotation of the photosensitive drum ; and an ACR executing unit is configured to calculate an offset of each color based on the detection result of the pattern detecting unit , and to correct a color registration error by use of the offset calculated. the pattern generating unit is configured to allow amounts of gap changes of a plurality of sub patterns, which together form the ACR pattern, to have an average value of about 0 such that a DC offset can be accurately determined, wherein the gap change of the plurality of sub patterns is caused by an AC component generated from a rotation of the photosensitive drum ; and an ACR executing unit is configured to calculate an offset of each color based on the detection result of the pattern detecting unit , and to correct a color registration error by use of the offset calculated. <EOS>