According to one embodiment, a decoder of a memory controller includes: a syndrome calculating unit configured to calculate a syndrome based upon a code word read from the memory; an error locator polynomial generating unit configured to generate an error locator polynomial based upon the syndrome, and to obtain a number of errors based upon the generated error locator polynomial; and an error location calculating unit configured to calculate an error location based upon the error locator polynomial, wherein the process of the error location calculating unit is not executed, when the number of errors is not less than the maximum number of bits that can be corrected by the error locator polynomial generating unit.