To attain proper shading correction with a simple configuration by scanning a reference image for several times and using its mean value as reference data.

A line sensor 10 scans a black/write reference board, an analog signal processing section 20 processes an image, an A/D converter 30 converts the result into a digital signal, and the digital signal is fed to a digital signal processing section 40. An address of each base area of R, G, B signals of a 1st pixel of an image signal is read from a memory 50 and the R, G, B signals are rewritten in each address. Such a processing is conducted for 1st, 5th, 9th and 13th line periods to obtain a signal for each pixel equipment to worth 4 lines. Then the data are read from a work area and accumulated to obtain a mean value of the data equipment to 4 lines for a period of 18th to 20th as the reference data. Thus, the reference data without fluctuation in the linear sensor and noise are obtained, while the capacity of the memory required for the processing is set constant.