1. An image reading apparatus, comprising: a reading unit configured to read an image of a target object; and a detector configured to detect an abnormal image that does not exist in the image and is caused by a foreign matter at a reading position of the reading unit, wherein: the reading unit comprises a plurality of line sensors each including a first light receiving element and a second light receiving element to form a plurality of light receiving elements arranged in a first direction, the plurality of line sensors being arranged at a predetermined interval in a second direction perpendicular to the first direction; the first light receiving element is configured to receive light of a first color and the second light receiving element is configured to receive light of a second color different from the first color; and the reading unit is configured to generate read data representing the image read by the respective line sensors, the detector comprises: an abnormal image detector configured to detect the abnormal image for each of colors based on pixels of the same color in different lines within the read data obtained by the respective plurality of line sensors; a combining unit configured to generate at least one of combined abnormal image information obtained by combining results of detecting the abnormal images of the respective colors in an area where the target object exists and combined abnormal image information obtained by combining results of detecting the abnormal images of the respective colors in an area where the target object does not exist; and a determination unit configured to determine, when the abnormal images exist within a range including the same position in the first direction in the area where the target object exists and the area where the target object does not exist based on the combined abnormal image information, that the abnormal image due to the foreign matter has occurred within the range.