A data compression system in a storage system compresses data with a first compression method to generate compressed data, determines whether a compression rate of the compressed data is better than a predetermined reference, outputs data obtained by compressing the data by the compression method having a better compression rate than that of the other compression method of the first compression method and a second compression method when it is determined that the compression rate is better than the reference, and outputs data obtained by compressing the data by the compression method having a worse compression rate than that of the other compression method of the first compression method and the second compression method when it is determined that the compression rate is equal to or worse than the reference.