To provide a reliable water level controller.

This water level controller comprises a changeover circuit 7 for outputting a water level retaining signal 6a that a hold circuit 6 outputs or an input water level signal 1a selectively, a rate-of-change check circuit 4 for detecting the rate of change of such input water level signals 1a and, when the detected rate of change exceeds a preset value, outputting a rate-of- change excess signal 7 that activates the changeover circuit 7, and an alarm adjuster 2 for comparing the water level signal 1a that the changeover circuit 7 outputs with a prescribed alarm setting value. With this constitution, the water level controller is prevented from running away accidentally.