In a picture processing apparatus, respective results of subtraction calculation between the relevant pixel and the reference pixels are obtained by a subtraction arithmetic circuit. The resultant differences are converted nonlinearly by a nonlinear conversion circuit, and thereafter a summation calculation of these resultant differences are performed by a first addition arithmetic circuit. The obtained result is added to data of the relevant pixel by a second addition arithmetic circuit; thereby achieving MTF correction. By adequately selecting the input-output characteristic of the nonlinear conversion circuit, a stable contour line enhancement is achievable even when noise is present in the picture signal.