PURPOSE: To prevent generation of arc discharge at an inlet bushing even if the liquid to be treated is boiled in a main body of a vessel by arranging the inlet bushing to be always in the liquid zone even if the liquid to be treated is boiled in the main body of the vessel.

CONSTITUTION: An inlet bushing 108 is provided to a position below the lowest sensible liquid level L by a liquid level sensor 110. When the liquid to be treated is boiled in the main body of the vessel 101 and the vapor is accumulated in the top of the main body of the vessel 101 to drop the liquid level to the lowest sensible liquid level L, a signal is outputted from the liquid level sensor 110 to lower the temp. in the main body of the vessel 101 and the pressure is raised simultaneously to cause condensation of the vapor and elevation of the liquid level. Accordingly, the inlet bushing 108 is not exposed to the vapor phase even in such state and generation of arc discharge is inhibited. Moreover, since the inlet bushing 108 is provided vertically to the wall of the main body of the vessel 101, generation of the arc discharge is made more difficult.