To provide a control method for generating image data with an increased S/N by extending an output range of a CCD sensor mounted on am image reader to read an image which generates image data adopting constitution of multi-value data and provides the data to a host device.

The control method for the image reader checks an output value of the CCD sensor mounted on the image reader to generate correction data to correct the output value of the CCD sensor, extends a range of a read output for image reading up to a nonlinear region to generate image data and applies linearity correction to the image data on the basis of the correction data. Further, a control program of the image reader includes a step for control with respect to generation of the correction data and a step of control for linearity correction.