A storage device for performing a reliability check by using error correction code (ECC) data is provided. The storage device includes a memory controller configured to detect the number of errors of second read data read out by a second read operation, based on ECC data of first read data read by a first read operation of a memory device. The memory controller includes a memory check circuit that includes a counter configured to count states of memory cells, a comparator configured to compare respective count numbers of the states with one another, and a register configured to store the number of errors based on a result of the comparison.