A memory system has a memory unit composed of a plurality of memory cells, a memory controller for controlling to read out and write data from and to the memory unit, and a host processor connected to the memory controller for reading out and writing data from and to the memory unit through the memory controller. The memory controller has a refresh controller for rewriting the data stored in the memory unit. The host processor has a determination unit for determining whether or not a refresh operation can be executed to the memory unit and a permission signal transmission unit for transmitting a refresh permission signal when it is determined that the refresh operation can be executed to the memory unit by the determination unit. The refresh controller controls the start of the refresh operation to the memory unit based on the refresh permission signal transmitted from the host processor.