PROBLEM TO BE SOLVED: To update software causing data type conversion (e.g. conversion from 8-bit type into 16-bit type) without affecting the operation after updating while continuing the system operation. SOLUTION: When data type conversion of both active/standby systems is not executed at switching of control means 102a, 102b of active/standby systems where data contents of both system common share memories 104a, 104b are coincident in the operation of the active system, a table A' of the standby system corresponding to a table A in the active system is entirely cleared, a table B is created in other areas, the old type data in the table A is converted into another type data and stored in the table B, after start of the data type conversion, difference data (old type data) written in the active system are stored in the table A' after the table A' is entirely cleared, as soon as the stored difference data are converted into another type data and stored in the table B, the corresponding data in the table A' are cleared and when the difference data are eliminated in this processing, the active/standby system is switched.
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