Frequently occurring DCT coefficients F(u, v) are predictable. Accordingly, values f.sub.u,v,F,(u,v) (x, y) for the frequently occurring DCT coefficients are calculated and stored, in advance, in a table. Among input DCT coefficients F(u, v), only non-zero DCT coefficients are processed. It is determined whether or not the table contains values f.sub.u,v,F(u,v) (x, y) for a given non-zero DCT coefficient. If the table contains the values, they are read out of the table, and if not, they are directly calculated. The read or calculated values are added to pixels (x, y), respectively, to provide pixel values (x, y).