PURPOSE: To prevent damage of a vibrator and other parts of the ultrasonic humidifier when liquid is not present in a tank, by using one of electrodes of the vibrator to also serve as a liquid level ditector so that the vibrator can be driven only when said electrode and other electrode are both submerged in the liquid.

CONSTITUTION: When an electrode 3 and a vibrator 1 are submerged in water, an electric current from an electric source 4 flows through a resistor R4, an electrode 3, a water, a vibrator electrode 2 and an R2, and a voltage between the electrode 3 and the earth becomes L level. Therefore, an output voltage e0 of a comparator including an OP amplifier 5 becomes the L level, the voltage e0 is divided to an R7 and an R8 and is applied to a transistor Tr2, but this is in an off condition. Consequently, a suitble bias voltage is applied to a Tr1 in an oscillation circuit, so that oscillations are effected by the vibrator 1 to atomise the water. When water is not present, said voltage e0 becomes an H level, the Tr2 is turned on to short-circuit a bias resistor R2, and the oscillation circuit is caused to be in an off condition.