1. A data compressing method including: a first step in which a first orthogonal transform is performed on a data so that an orthogonal transform data is generated; and a subsequent processing step executed subsequent to the first step in which an inverse transform equivalent to a decoding process of the orthogonal transform data is executed on the orthogonal transform data, wherein: the subsequent processing step includes a second step for processing an alternate-current component of the orthogonal transform data, a third step for processing a direct-current component of the orthogonal transform data, and a finalizing step, the second step and the third step are performed in parallel, the second step includes a first inverse orthogonal transform corresponding to the first orthogonal transform in the first step, the first inverse orthogonal transform being an inverse orthogonal transform in which the direct-current component of the orthogonal transform data is zero, the third step includes a second orthogonal transform for the direct-current component of the orthogonal transform data, and a second inverse orthogonal transform corresponding to the second orthogonal transform, the first step comprises plural first processing slices, the second step for the alternate-current component comprises plural second processing slices, a process for each of the plural second processing slices starts at an end of a corresponding process for each of the plural first processing slices, the third step starts before completion of the last slice of the plural first processing slices, and the finalizing step creates a sum of a result of each of the plural second processing slices and a result of the third step.