1. A resolution changing method for changing the resolution of image data subjected to a predetermined image processing involving block split, the method comprising: a reference pixel selection step of selecting a plurality of reference pixels to be supplied to an operation of an interpolation pixel from the image data; a determination step of determining whether or not the plurality of reference pixels include a boundary of the block; and an interpolation pixel data computation step of computing pixel data of an interpolation pixel by performing a first kind of interpolation processing by the use of a plurality of pixel data of the reference pixels when the plurality of reference pixels do not include the boundary of the block and of computing pixel data of an interpolation pixel by performing a second kind of interpolation processing by the use of a plurality of pixel data of the reference pixels when the plurality of reference pixels include the boundary of the block, wherein the second kind of interpolation processing produces a smaller effect of enhancing an edge of an image as compared with the first kind of interpolation processing.