PURPOSE: To decrease the quantity of droplets which are scattered from the free surface and released to the outside, associated with air current in a device where liquid is heated and evaporated in a vessel, such as a distiller and a concentrator.

CONSTITUTION: When a heating source is near the central part of the vessel in a boiling vessel 1, after bubbles 5 are collected by a bubble collector 2, they are conducted to near the wall of the free surface inside the boiling vessel 1 by using a bubble conduit 3. And when the heating source is on or near the wall of the boiling vessel 1, bubbles 5 are conducted to near the wall of the free surface inside the boiling vessel 1 by using a bubble movement restraint cylinder. By releasing the bubbles 5 mainly near the wall of the boiling vessel 1, the scattering of the droplets 6 due to the burst of the bubbles 5, etc., takes place mainly near the wall of the boiling vessel 1. Thereby most of the droplets scattered from the free surface collide with the wall of the boiling vessel and are condensed, causing the quantity of the droplets released to the outside of the boiling vessel to be decreased.