PROBLEM TO BE SOLVED: To obtain a large capacity data processing method reducing memory capacity without losing the speed at which the display on a screen is performed and the plant monitoring control system using the method. SOLUTION: In this system, a central processing unit transferring collected data from a buffer memory 11 to a hard disk 13 when the storage in the buffer memory 11 of collected data collected via a network reaches prescribed quantity based on preliminarily set data and starting the storage in a buffer memory 12 is provided, and the buffer memories 11 and 12 are used by a switch. Thus, data collection can be performed without omission, memory capacity can be reduced and large capacity data processing can be performed at high speed.
