PURPOSE: To provide more real pictures by detecting the area of a picture buffer corresponding to the contour part of a pattern of/on an object using the differentiated values of the picture buffer and a Z buffer and filtering it.

CONSTITUTION: When picture generation is completed, a differential filter 6 reads the picture buffer 3, detects a part where the fluctuation of the brightness is large and raises a bit 1 at a position inside a mask buffer 7 corresponding to the part. Then, the differential filter 6 reads the Z buffer 4, similarly detects the part where the fluctuation of a Z value is large and raises the bit 1 at the position inside the mask buffer 7 corresponding to the part. When contour detection is completed, a controller 1 selects a frame buffer 10 by controlling a switch 9 and an averaging filter 8 reads picture data from the picture buffer 3 and mask data from the mask buffer 7, exchanging them with a weighing average value with the adjacent picture elements of the picture data for which a mask value is '1' and writes a picture element value in the frame buffer 10.