PURPOSE: To improve the resolution in a subscanning by giving the pixel data of a current scanning line and the pixel data of a just preceding scanning line to a conversion data storage section as a table address so as to output conversion pixel data.

CONSTITUTION: This device is provided with an A/D converter 2, a storage section 4 storing the pixel data of a just preceding read line, and a conversion data storage section 6 storing in advance a conversion value used for conversion of the pixel data. Then the pixel data of the current scanning line and the pixel data of the just preceding scanning line are imparted to the conversion data storage section 6 as a table address to output the conversion pixel data. Thus, correction is implemented by taking the difference of density and the pixel data of the scanning line just preceding to the current scanning line into account. Thus, a level with a sufficient binary value is obtained at a changing point from a white level to a black level or vice versa and the resolution in the subscanning being the original carrying direction is improved.