1. A data compressing method including: a first step in which a data is orthogonally transformed so that an orthogonal transform data is generated; and a processing step executed subsequent to the first step, wherein the subsequent processing step includes a second step in which an inverse transform equivalent to a decoding process of the orthogonal transform data is executed on the orthogonal transform data, and the subsequent processing step is divided into a processing step for an alternate-current component of the orthogonal transform data and a processing step for a direct-current component of the orthogonal transform data.