An apparatus includes an image sensor that has a plurality of sensor chips having photoelectric conversion elements and that outputs pixel signals from a plurality of output channels; identifying means that identifies a width of a document to be read by the image sensor; and signal control means that controls signals to be output from the image sensor. In the image sensor, different sensor chips at an interval of the number of channels of the output channels are connected to each output channel, and the signal control means controls output of a pixel signal of each sensor chip based on the identified document width.