1. A resolution changing method for changing the resolution of image data obtained by decoding an encoded data which is encoded by orthogonal transformation encoding processing in which an image data is split into blocks of a predetermined size, 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 an area comprising the plurality of reference pixels exists across 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 if the area comprising the plurality of reference pixels does not exist across 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 if the area comprising the plurality of reference pixels exists across the boundary of the block, wherein the second kind of interpolation processing produces a smaller effect of enhancing an edge of an image and a larger effect of smoothing a discontinuous portion of the boundary of the block as compared with the first kind of interpolation processing.