Documents used for process control can be managed and maintained from a document server and a terminal. In order to update the documents quickly so that an operated process is always kept in the most preferable state of operation, the document server stores and manages production documents used for actually controlling and monitoring a control system plant, whereas the terminal system is used to refer and edit plant documents. The terminal can synchronize contents of the plant documents in the document server with those in the control system before storing the synchronized plant documents in the terminal.