A memory system includes a nonvolatile semiconductor memory and a memory controller circuit. The memory controller circuit selects first and second blocks of the nonvolatile semiconductor memory, the first block being a garbage collection target block, the second block being a wear leveling target block or a refresh target block, relocates first data which is valid data stored in the first block in a series of write operations to a third block including first and second write operations, the third block being a block of the nonvolatile semiconductor memory having a free region, and relocates second data which is valid data stored in the second block in a series of write operations to a fourth block including a third write operation, the fourth block having a free region and being different from the third block, wherein the third write operation is performed between the first and second write operations.