A plurality of light-receiving elements that are arranged in two rows are provided on a light-receiving surface of a detector. A slit image formed on this detector. One group of a plurality of the light-receiving elements are arranged consecutively in a displacement direction of the slit image to form a row (one light-receiving elements row), and another group of a plurality of the light-receiving elements are also arranged consecutively in the displacement direction of the slit image to form a row (another light-receiving elements row). The one light-receiving elements row and the other light-receiving elements row are in contact with each other.