Proposed is a storage system and a data guarantee method capable of guaranteeing and verifying the reliability of data in a short period of time even with the heavy usage of inexpensive hard disk drives. This storage system having a storage apparatus that provides a storage extent for reading and writing data from and in a host system, and a storage controller for controlling the storage apparatus includes a first generation unit for generating a guarantee number in data from a host system to be stored in the storage apparatus showing the guarantee of the data, a second generation unit for generating a guarantee number showing the guarantee of parity data corresponding to data from the host system, and a verification unit for verifying the guarantee number showing the guarantee of data from the host system and the guarantee number showing the guarantee of the parity data.