PURPOSE:To decide the density of picture elements for processing time similar to conventional RMW(read/modify/write) processing without lowering picture quality. CONSTITUTION:An LUT circuit 42 calculates successively the picture element density from density data, area rate K and density Dm by using RMW processing, buffers 43a-43c output the density of respective left, objective, and right picture elements as outputs A, B and C, and an arithmetic part 44 compares the output B with the other picture element, decides density Dn of the objective picture element and outputs it to a frame memory. Namely, the density is compared with the right and left picture elements or five picture elements including three picture elements in read adjacent lines and when the density is lower than the other density, the density of any one of adjacent picture elements or the average value of outputs A and C is defined as Dn. When the density is out of the range of outputs A and C, the intermediate value is defined as Dn. In the other case, the output B is defined as Dn. Thus, the picture quality is not lowered by running out of the density of an edge part from the area density on both of sides.