When a monochromatic mode is set, a CPU outputs a SW signal and executes a control to input an output from a monochromatic reading line sensor to a CCD signal processing section. Then, after a black level adjustment period ends and a black reference level reaches a desired value, the CPU starts reading of a black reference signal for shading correction. When a color mode is set, the CPU outputs a SW signal and executes a control to input an output from a color reading line sensor to the CCD signal processing section. After the black level adjustment period ends and the black reference level reaches a desired value, the CPU starts reading of a black reference signal for shading correction.