To provide a scanner for reducing time required for reading operation of a monochrome manuscript by a scanner having a plurality of line sensors.

The scanner has N image sensors that are arranged at a line interval that is M times larger than a pixel size. In the scanner, the image sensors and the manuscript move relatively at a specific sub scanning speed in the reading operation of color images, and each image sensor reads the image of the sub scanning position. In the scanner, the sub scanning speed in a monochrome image-reading operation is switched to N times larger than the sub scanning speed in a color image-reading operation. However, in this case, N and M are prime integers each other.