PURPOSE: To make it possible to accurately carry out measuring feature quantity and pattern matching by emphasizing edge to eliminate the effect of illumination fluctuation and realizing preprocessing where noise component is not emphasized.

CONSTITUTION: The present image processor 40 includes a preprocessing circuit 42 constituted of an A/D converter 47, a differentiating arithmetic section 48, a multiplier 49, a divider 50, and a adder 51. In the differentiating arithmetic section 48 a gradation image signal of digital variable is inputted, and the first order differentiating arithmetic section 52 calculates an absolute value of the first order differentiation and the second order differentiating arithmetic section 53 the second order differentiated value, respectively. The multiplier 49 carries out multiplication of each differentiation arithmetic output, the divider 50 divides a multiplication output by a constant, and the adder 51 adds the division output to the gradation image signal, thereby generating an output signal to a binarization circuit 43.