1. An image processor comprising: a detection unit that detects a plurality of specified images each having a shape from an image read out by an image reading apparatus; a first angle determination unit that determines a rotation angle of each of the plurality of specified images detected by the detection unit by a first method on the basis of presence or absence of other specified image in a position determined in advance with respect to each of the plurality of specified images; a second angle determination unit that determines a rotation angle of each of the plurality of specified images detected by the detection unit by a second method on the basis of whether or not a pattern of each of the plurality of specified images and a pattern of an image representing a predetermined rotation angle match each other; and a processing target determination unit that determines whether or not each of the plurality of specified images is to be subjected to a processing on the basis of the rotation angle determined by the first angle determination unit and the rotation angle determined by the second angle determination unit.