There is a problem that the contact-type image sensor, for adoption in an image input/output apparatus to input/output the image of a large-sized document, such as A0 or A1 size, is weak in lengthwise rigidity and readily deflected at its lengthwise center by its own weight. Accordingly, by attaching a deformation rectifier to the image sensor in a lengthwise direction thereof, the contact type image sensor is reinforced in lengthwise rigidity. The deformation rectifier reinforces the rigidity of the contact type image sensor, thereby preventing the contact type image sensor from deflecting vertically relative to its lengthwise direction and keeping constant the focal length between the surface of the document to be read and the sensor IC.