PURPOSE: To make is possible to obtain smooth output without developing the breakage of fine line, realize high processing speed due to comparatively simple operation and further prevent the lowering of contrast of character and line drawing at the conversion of two-valued character and line drawing data to many-valued image data having low image element density.

CONSTITUTION: At respective setting parts 10 and 12, the conversion multiplying factor of image element density and arbitrary number of image elements at arbitrary positions in a reference area, in which (m×n) image elements are included, near a sampling point as an image element reference pattern are set. From character and line drawing data consisting of two-valued data or the minimum and maximum values obtainable among many-valued data, a reference area is selected in response to the conversion multiplying factor and the image element reference pattern, so as to select reference image elements from the reference area in response to the image element reference pattern. At an operational or tabularly converting means 20, the converted image element values of the reference area are obtained from the adding values of the image element values of the reference image elements through arithmetic operation or tabular conversion.