PURPOSE: To enable an operator to easily and speedily judge variation in the process quantity of a liquid level.

CONSTITUTION: A sensor 22 which detects the water level in a tank 201 to be monitored is installed in the tank 201. A process input means 23 inputs the process signal from the sensor 22. A variation decision part 27 decides whether or not the inputted process quantity varies. A display control means 25 generates data for bar graph display on the basis of the variation/ nonvariation state of the process quantity decided by the variation decision part 27 and the process quantity. When the variation is decided, the tip of the bar graph is formed in a wave shape. A CRT 26 displays the bar graph data generated by the display control means 25.