To realize efficient evaporation-concentration with an apparatus smaller in size and inexpensive in production cost in such constitution that a bath containing a heater is installed on the outside of a storage tank to store a portion of condensate and that the condensate in the storage tank is heated up through the heated condensate in the bath.

A vaporizer 100 is mainly comprised of a storage tank 110 which stores a proper quantity of liquid, an auxiliary device 113 which makes the liquid move into and out of the storage tank 110 by motions such as a circulating motion and a reciprocating motion, and a heater H such as an oil- enclosed heater and a cast heater installed at the bottom or in the lower part outside the storage tank 110. The heater H heats up the liquid in the storage tank 110 through a heat medium consisting, for example, a condensate which is stored in a bath installed on the outside of the storage tank 110. This allows the apparatus to obtain a larger heat transfer area to promote uniform heating and to prevent local heating in the storage tank 110, as compared with such a constitution that the heater H heats up the liquid directly.