The present invention relates to an image reading apparatus in which a structure is simple and in which the size of a document can be detected in a short time, wherein the levels of signals, which are outputted from photoelectric elements within a plurality of sensing regions on a light-receiving portion of a line sensor correspond to a pair of nearby regions (#1 and #2, #3 and #4, #5 and #6, and #7 and #8) which hold each edge (e.g., edges 64A, 64B, 64C in FIG. 5) of the plurality of documents of different sizes disposed on a platen glass, are compared to a threshold value, or alternatively, the change in the levels of signals which are outputted from the photoelectric elements within a plurality of sensing regions (e.g., #1, #2 and #3 in FIG. 11) continuously cross each edge of the plurality documents of different sizes is obtained in order to determine the size of the document. An optical sensor which detects the existence of the document at a predetermined position (#0) on the platen glass is provided so that when the document is disposed in a transverse direction (SEF), the documents exists, and when the document is disposed in a longitudinal direction (LEF), the document does not exist. Accordingly, the direction of the document is determined. Due to this structure, the size and direction of the document disposed on the transparent member are determined.