PURPOSE: To represent the contour vividly by comparing a dispersion value with the referent dispersion value by a comparator, carrying out stressing process only in case the current dispersion value is greater the reference dispersion value, and thereby representing thick parts of a diagnostic image thick and thin parts thin.

CONSTITUTION: Information stored in a memory 14 is supplied one after another to a shift register 20 in an image processing device 18, and the picture element mean value is calculated by circuits from an adding/subtracting device 22 till a subtracting device 26. The square mean value z2 is calculated through circuits from a shift register 32 till another subtracting device 38, and the output of a square multiplying device 28 is subtracted from the output of the subtracting device 38 by a subtractor 40 to compute a dispersion value σ. Then a comparator 44 judges whether this dispersion value σ is greater than the reference dispersion value σc, and if σ>σc the factor K is calculated while if σ≤σc the factor K becomes zero. In case the attention picture element value is greater than the picture element mean value, it is stressed still larger, and in case the attention picture element value is smaller than the picture element mean, it is stressed still smaller.