Interpolation data for a missing pixel are obtained (4) after a correction (3) for variations in the characteristics of the imaging devices (8(1), 8(2)). The correction for variations in characteristics includes, for example, a black level correction (9) and a sensitivity correction (10). The interpolation includes, for example, interpolation calculations (14(1), 14(2)) such that a mean value of a plurality of pixels including the missing pixel and a mean value of a plurality of pixels not including the missing pixel become equal and an interpolation calculation (14(0)) that takes the mean of the pixels to the right and left of the missing pixel, the calculation with the least error being selected (13, 15). Thus missing pixels in highly periodic images can be properly interpolated, and interpolation can be performed with high precision even when there are variations in the sensitivity characteristics of the individual imaging devices constituting the contact image sensor.