Memory system

A memory system includes a nonvolatile memory including blocks, and a memory controller. The memory controller is configured to set each of the blocks to be in one of a plurality of states, including first, second, third, and fourth states. The memory controller is configured to detect a predetermined condition related to at least one of an amount of data being written into blocks in the first state and state transition of a block, upon detection of the predetermined condition, determine a maximum number of blocks to be in the fourth state based on a length of time during which each block in the fourth state has been in the fourth state, and perform an erase operation to cause one or more blocks in the third state to transition to the fourth state when a current number of blocks in the fourth state is less than the maximum number.