A numerical controller includes: a monitor unit that monitors a water level; a backup unit that performs power supply when a main power source is turned off; and a cutoff circuit that cuts off the power supply, and when the water level monitored by the monitor unit detects a predetermined water level, the power source of the backup unit from an external power source means is cut off. Thus, the power supply is cut off before the flooding occurs in the numerical controller, which makes it possible to reduce the damage caused by electrolytic corrosion.