IMAGE PROCESSING APPARATUS

During pre-scanning, a CPU calculates a gradation data width corresponding to the number of pixels which are 0.3% of the total pixels from a minimum value in a frequency distribution curve of gradation data of all pixels; calculates effective minimum and maximum values from the minimum and maximum values of the gradation data of all pixels and the gradation data width. A gradation correction i performed in a gradation correcting device during main scanning using following equation: EQU f(x)=[{255-(S+L)}x.sup.2 +(S+L).sup.2 x-SL(S+L)-255S.sup.2 ]/(L.sup.2 -S.sup.2) where PA1 x: gradation data before the correction PA1 S: effective minimum value PA1 L: effective maximum value PA1 f(x): gradation data after the correction.