To easily make comparison and examination of both a measured value and an event.

A monitor device 3 is provided with a means for preparing a first window where the details of the history of a measured value are displayed as a list in response to the display request of an operator, a means for preparing a second window where the details of the occurrence history of an event are displayed as a list in response to a display request and a means for simultaneously displaying the prepared first window and second window on the operation monitor screen.