An image forming apparatus of the laser beam type, comprising: a reception unit adapted to receive, from an external apparatus, first corrected data, which is obtained by correcting, based on deformation information generated upon formation of image data of an original image, pixels in a vertical direction of the image data in a first unit, wherein the first unit is an unit of a data transfer in said external apparatus and a block unit of predetermined pixels included in the image data; the reception unit being further adapted to receive, from the external apparatus, horizontal correction information, which is based on the deformation information, required to correct pixels in a horizontal direction of the image data; the reception unit being further adapted to receive, from the external apparatus, vertical correction information required to correct pixels, which cannot be corrected in the first unit in the vertical direction of the image data based on the deformation information, in a second unit, wherein the second unit is a unit of one pixel of the image data; a correction unit adapted to generate final corrected data of the image data by correcting the first corrected data using the horizontal correction information and the vertical correction information; and an image forming unit adapted to form an image based on the final corrected data, wherein said correction unit comprises: a horizontal independent scaling unit adapted to generate second corrected data by correcting the first corrected data using the horizontal correction information; a temporary storage unit adapted to temporarily store the second corrected data; and a selection unit adapted to generate the final corrected data by selecting, based on the vertical correction information, one of data of a current line acquired without storing the second corrected data in said temporary storage unit, and data one line before, which has undergone delay processing by storing the second corrected data in said temporary storage unit, wherein the image forming apparatus further comprises a transmission unit adapted to transmit the deformation information to the external apparatus before said reception unit receives the first corrected data from the external apparatus. a reception unit adapted to receive, from an external apparatus, first corrected data, which is obtained by correcting, based on deformation information generated upon formation of image data of an original image, pixels in a vertical direction of the image data in a first unit, wherein the first unit is an unit of a data transfer in said external apparatus and a block unit of predetermined pixels included in the image data; the reception unit being further adapted to receive, from the external apparatus, horizontal correction information, which is based on the deformation information, required to correct pixels in a horizontal direction of the image data; the reception unit being further adapted to receive, from the external apparatus, vertical correction information required to correct pixels, which cannot be corrected in the first unit in the vertical direction of the image data based on the deformation information, in a second unit, wherein the second unit is a unit of one pixel of the image data; a correction unit adapted to generate final corrected data of the image data by correcting the first corrected data using the horizontal correction information and the vertical correction information; and an image forming unit adapted to form an image based on the final corrected data, wherein said correction unit comprises: a horizontal independent scaling unit adapted to generate second corrected data by correcting the first corrected data using the horizontal correction information; a temporary storage unit adapted to temporarily store the second corrected data; and a selection unit adapted to generate the final corrected data by selecting, based on the vertical correction information, one of data of a current line acquired without storing the second corrected data in said temporary storage unit, and data one line before, which has undergone delay processing by storing the second corrected data in said temporary storage unit, wherein the image forming apparatus further comprises a transmission unit adapted to transmit the deformation information to the external apparatus before said reception unit receives the first corrected data from the external apparatus. a horizontal independent scaling unit adapted to generate second corrected data by correcting the first corrected data using the horizontal correction information; a temporary storage unit adapted to temporarily store the second corrected data; and a selection unit adapted to generate the final corrected data by selecting, based on the vertical correction information, one of data of a current line acquired without storing the second corrected data in said temporary storage unit, and data one line before, which has undergone delay processing by storing the second corrected data in said temporary storage unit, wherein the image forming apparatus further comprises a transmission unit adapted to transmit the deformation information to the external apparatus before said reception unit receives the first corrected data from the external apparatus.