A data management system and data management method capable of inhibiting an increase of I/Os caused by deduplication are provided.Data blocks are managed by grouping them; a plurality of chunks belonging to the same group after deduplication are collectively managed as a chunk data set; and if a host system issues a data block deletion request, garbage collection of the relevant chunk data set is performed based on the number of times of references of the chunk data set and the chunk data set after the garbage collection is managed by overwriting the existing chunk data set with a new chunk data set composed of only chunks of data blocks which are not deleted.