An original document detection apparatus comprising: an imaging unit adapted to acquire a background image by imaging a background of an original document in a state where the original document is not arranged, and acquires an original document including background image by imaging the same range as the background image in a state where the original document is arranged on the background; a difference image generation unit adapted to generate a difference image obtained by subtracting gradation values of corresponding pixels of the background image from gradation values of pixels of the original document including background image, respectively; characterized by: an inverse difference image generation unit adapted to generate an inverse difference image obtained by subtracting the gradation values of the corresponding pixels of the original document including background image from the gradation values of the pixels of the background image, respectively; a sum image generation unit adapted to generate a sum image obtained by summing gradation values of pixels of the difference image and gradation values of corresponding pixels of the inverse difference image; and an original document region detection unit adapted to detect a region of the original document based on the sum image, wherein in a case where a gradation value of a generated image is negative, this gradation value is set tc "0". an imaging unit adapted to acquire a background image by imaging a background of an original document in a state where the original document is not arranged, and acquires an original document including background image by imaging the same range as the background image in a state where the original document is arranged on the background; a difference image generation unit adapted to generate a difference image obtained by subtracting gradation values of corresponding pixels of the background image from gradation values of pixels of the original document including background image, respectively; characterized by: an inverse difference image generation unit adapted to generate an inverse difference image obtained by subtracting the gradation values of the corresponding pixels of the original document including background image from the gradation values of the pixels of the background image, respectively; a sum image generation unit adapted to generate a sum image obtained by summing gradation values of pixels of the difference image and gradation values of corresponding pixels of the inverse difference image; and an original document region detection unit adapted to detect a region of the original document based on the sum image, wherein in a case where a gradation value of a generated image is negative, this gradation value is set tc "0". an inverse difference image generation unit adapted to generate an inverse difference image obtained by subtracting the gradation values of the corresponding pixels of the original document including background image from the gradation values of the pixels of the background image, respectively; a sum image generation unit adapted to generate a sum image obtained by summing gradation values of pixels of the difference image and gradation values of corresponding pixels of the inverse difference image; and an original document region detection unit adapted to detect a region of the original document based on the sum image, wherein in a case where a gradation value of a generated image is negative, this gradation value is set tc "0".