According to one embodiment, a storage device includes a nonvolatile memory, a controller configured to copy data stored in a first page in a first block to a second page in a second block, and an ECC circuit. The controller reads data from a part of the first page by using an upper limit voltage and lower limit voltage, performs a direct copy operation in the nonvolatile memory without via the ECC circuit if the number of error cells having threshold voltages higher than the lower limit voltage and lower than or equal to the upper limit voltage is less than or equal to a specified value, and performs error correction by using the ECC circuit if the number of error cells exceeds the specified value.