To prevent a carry over due to foaming by providing a foam detector positioned above the surface of a liquid to be distilled and lowering the output of a heater when the foam generated from the liquid to be distilled is detected at the time of the distillation of the liquid to be distilled in a distillation vessel by boiling with a heater.

At the time of distilling a foamable waste liquid, a liquid 4 to be distilled, which is a foamable waste liquid, is charged to the distillation vessel 1. And, the liquid 4 to be distilled is heated by the heater 5, and generates foam 16 at the time of starting to be boiled and the growth of the foam 16 is increased at the time of being concentrated to a certain degree. When the foam 16 is brought into contact with the detector 6 at this time, current is passed through a lead wire 12. The foam is suppressed by detecting the change of the current with a current monitor 13 and sending the signal to a heater control circuit 14 to lower the output of the heater 5. After that, if the foam 16 is decreased and the conduction of the foam detector 6 is eliminated, the distillation is carried out with the increase of the output to the heater 5.