An image reading device comprising: a document reading unit configured to collectively read a plurality of source documents (D21, D22, D23, D24) placed on a document table; an individual image cropper that crops an independent individual image of each of the source documents, from image data representing one face of the source documents and image data representing another face of the source documents, acquired after the source documents are reversed, both of the image data having been acquired through a reading operation of the document reading unit; and a document position detector that detects a position of each of the individual images cropped by the individual image cropper from both of the image data; characterized by: a gravity center detector that detects, with respect to each of the both of the image data, a center of gravity (G1, G2) of a set composed of the individual images, on a basis of a position of each of the individual images detected by the document position detector from both of the image data; a second relative position detector that detects, with respect to each of the individual images, a relative position indicating a positional relation relative to the center of gravity (G1, G2) detected by the gravity center detector in both of the image data; and a second pair identifier that performs pair identification including identifying, as an image pair representing front and back faces of a same source document, a pair of the individual images, the respective relative positions of which, from the center of gravity (G1, G2) in one of the both image data and from the center of gravity (G1, G2) in the other one of the both image data, are closest to each other. a document reading unit configured to collectively read a plurality of source documents (D21, D22, D23, D24) placed on a document table; an individual image cropper that crops an independent individual image of each of the source documents, from image data representing one face of the source documents and image data representing another face of the source documents, acquired after the source documents are reversed, both of the image data having been acquired through a reading operation of the document reading unit; and a document position detector that detects a position of each of the individual images cropped by the individual image cropper from both of the image data; characterized by: a gravity center detector that detects, with respect to each of the both of the image data, a center of gravity (G1, G2) of a set composed of the individual images, on a basis of a position of each of the individual images detected by the document position detector from both of the image data; a second relative position detector that detects, with respect to each of the individual images, a relative position indicating a positional relation relative to the center of gravity (G1, G2) detected by the gravity center detector in both of the image data; and a second pair identifier that performs pair identification including identifying, as an image pair representing front and back faces of a same source document, a pair of the individual images, the respective relative positions of which, from the center of gravity (G1, G2) in one of the both image data and from the center of gravity (G1, G2) in the other one of the both image data, are closest to each other. a gravity center detector that detects, with respect to each of the both of the image data, a center of gravity (G1, G2) of a set composed of the individual images, on a basis of a position of each of the individual images detected by the document position detector from both of the image data; a second relative position detector that detects, with respect to each of the individual images, a relative position indicating a positional relation relative to the center of gravity (G1, G2) detected by the gravity center detector in both of the image data; and a second pair identifier that performs pair identification including identifying, as an image pair representing front and back faces of a same source document, a pair of the individual images, the respective relative positions of which, from the center of gravity (G1, G2) in one of the both image data and from the center of gravity (G1, G2) in the other one of the both image data, are closest to each other.