PURPOSE: To improve reliability of the whole system by preventing data missing or the like in advance without increasing the load on a data processing job by a data processing unit.

CONSTITUTION: A data collection device 11 outputs the same data to two transmission lines 12a, 12b being an active system transmission line and a standby system transmission line, a transmission interface section 13a of a data processing unit 13 receives data sent on the two transmission lines and stores the data and checks the normality of the data of the active system transmission line in the two stored data to transfer the data to a data memory. When the result of the check by a 1st check means indicates an error, the normality of data of the other standby system transmission line to be stored is checked and transferred to a data memory. When the result of the check by a 2nd check means indicates a normal state, the designation of the active system transmission line and the standby system transmission line is revised.