An image-reading device includes a plurality of image sensor chips, a switch, an output selector and a controller. The plurality of image sensor chips is juxtaposed along a line extending in a widthwise direction of an original document carrying images thereon. Each image sensor chip includes a plurality of image sensors, reads the images on the original document, and generates image signals. The switch is configured to combine at least two image sensor chips into one virtual image sensor chip. The output selector allows the image signals from the virtual image sensor chip and remaining image sensor chip to be outputted. The controller controls the switch depending on the width of the original document, and controls, depending on the width of the original document, the output selector to allow the image signals to be outputted.