Provided are a storage system, a storage controller and a data compression method that will not deteriorate the performance of a storage system by adjusting the compression processing time required for compressing data. This storage controller includes a statistical information management unit for managing a throughput value decided based on statistical information of previously compressed file data as a system performance value, a mode setting unit for setting a compression mode of either a system performance priority mode that terminates compression of the file data and gives priority to system performance or a compression priority mode that gives priority to compression of the file data based on a comparison between a system performance target value preset as a target value and the system performance value, and a compression controller for compressing the file data based on the compression mode set with the mode setting unit.