PURPOSE: To suppress the increase of data accompanying the increase of the number of monitoring object processes.

CONSTITUTION: A data compression device 5 extracts the fluctuation of process data, performs Huffman encoding and compresses the data. Then, while the data are transferred through a network to a data preservation device 4 and preserved, a data restoration device 6 restores the transferred compressed data and the compressed data preserved in the data preservation device 4 as the process data for monitoring. As a result, since the process data can be compressed even when the number of the processes of monitoring objects increases and the process data increase accompanying it, the transfer efficiency of the data is improved and a data preservation area can be reduced.