A MAC address management method for use in a computer system having a disk device to store a MAC address and assigns the MAC address to an open network adaptor (ONA) printed board, which has a memory retaining a MAC address and an open network adaptor, mounted into the computer system. The METHOD includes: reading, when an ONA printed board is mounted into the computer system for the first time, the MAC address from the memory on the ONA printed board and storing the MAC address in the disk device and invalidating the MAC address; and validating, if a migration instruction is inputted, the MAC address stored in the memory of the ONA printed board mounted in the computer system.