PURPOSE: To expand an object 2 without changing the density state of the presence area of the object before an expansion processing of the object by performing the expansion processing only on the picture elements of a part judged that the object is not present by a threshold value for discrimination the object from a background.

CONSTITUTION: In pictures to be a processing object obtained by picking up the images of parts, the density data of the respective picture elements are generated, a density value for the picture element of interest is compared with the threshold value for discriminating the object 80 from the background 85 and partial pictures 10 are set only in the case of being equal to or less than the threshold value, that is, only in the case of the picture element where the object 80 is not present. Then, in a selective expansion processing process, in the case of the partial picture 10, '3' of the maximum value among the density values '1', '3', '3' and '3' of the respective picture elements is selected and an output density value '3' for an output picture element corresponding to the picture element 11 of interest in the output picture elements is set as output density data. Then, whether or not is is a final picture element of interest is judged, the picture element of interest is moved when it is not the final picture element under consideration, similar processings are repeated until the final picture element of interest and the expansion processing is performed only for the outside of the object.