An image reading apparatus enabling a user to flexibly select one of plural reading modes, for example, for continuously reading two-sided images of stacked document sheets or reading pages of a book or a large document exceeding a platen. In a first reading mode, a reading unit scans a document and reads a document image. In a second reading mode, a stationary reading unit and the reading unit read an image of a document fed from a document feeder section. The reading unit is provided in a flatbed reading section which is detachable from an apparatus body. The flatbed reading section, when detached from the apparatus body, can be used as a flatbed scanner.