After shadow bits in digital signal of image scanned are sent to computer, subsequent processing and output are carried out for the digital signal. The invented image processing method includes at least following steps: converting analog signal of image in scanner to digital signal; next, enhancing shadow bits by using first gamma curve; through bus, transferring digital signal to computer; then, reversing digital signal by using second gamma curve; carrying out image processing for digital signal; finally, carrying out image conversion for digital signal and outputting converted result by using third gamma curve.