A data compressing device according to an embodiment includes a data cutting unit configured to divide continuously inputted data into W-bit data blocks and to output the data blocks in segments such that each of the segments is composed of N data blocks, and a compression-method determining unit configured to select, as a compression portion for each of the segments, a run length system, a flag system, or no compression, according to a ratio of data blocks of specific data in any of the segments. The data compressing device further includes an RL compression unit configured to execute, on any of the segments, a run length system of storing a consecutive amount of the specific data into compressed data, and a flag compression unit configured to execute, on any of the segments, a flag system of storing positional information of the specific data into compressed data.