To provide a monitor device which can vary the display power of a trend graph automatically and speedily copes with the variation state so that the variation state of process data can be displayed when the process data exceeds the display upper or lower limit of the trend graph.

This monitor device is provided with a range specifying means 6 which specifies the display range of one or more piece of process data to be monitored, a decision means 7 which decides whether obtained process data can be put in the display range specified by the range specifying means 6, a display range varying means 8 which widen the display range so that the process data can be put in it when the deciding decision means 7 decides that the data can not be put in the range, and display means 3 and 4 which displays the process data with the display range varied by the display range varying means 8.