To provide a CPU board which can guarantee restarting even if abnormality such as a power break occurs during the downloading of a program or data by providing storage media of redundant constitution.

The CPU board 1 comprises storage media 1.11 and 2.12 having redundant constitution, a program operation area 13, a nonvolatile area 14 stored with information showing whether the storage media 1.11 and 2.12 are in use or standby mode and status information, and a control part 15 which controls the operation of the CPU board 1, e.g. controls the status of the storage media 1.11 and 2.12. Then the control part 15 performs control so that one of the storage media 1.11 and 2.12 is always a read-only medium. The restarting of the CPU board 1 is guaranteed with information stored on the read-only storage medium.