There are provided a data compression apparatus and a data compression program capable of performing new and preferable compression processing applicable to compression of CT data. They are provided with: a thinning processing section which creates, by cyclically thinning out a numeric value from the sequence of numeric values constituting the data to be compressed, first data to be compressed which is constituted by a sequence of the numeric values taken out from the data to be compressed by the thinning out, and second data to be compressed which is constituted by a sequence of the remaining numeric values; a lossless compression section which performs lossless compression processing for the first data to be compressed which has been created by the thinning processing section; and a lossy compression section which performs lossy compression processing for the second data to be compressed which has been created by the thinning processing section.