PURPOSE: To ensure the synchronism of process data by inputting process data in accordance with a timing clock and adding and transmitting timing counter values.

CONSTITUTION: A node computer 3A fetches process data PV6V and PV6B and the timing counter value T1 added to the data. A centralized monitoring computer 1 recognizes the termination of the cycle of the timing clock, then, fetches transmitted data from the node computer by a data fetching processing P1A and stores process data for respective node computers as a node 1 and a node 2. Here, the timing counter value T1 added to process data is referred to and the data groups of the same timing counter values T1 and T2 are divided by a processing P1B. Consequently, the synchronism of process data can be ensured even if the time difference of process data in the node computer and the time difference of process data between the node computers occur.