To effectively evaporate and dehydrate a material to be concentrated by providing an exhaust pipe with an exhaust branch pipe.

A rotor 2 is subjected to rotary driving and also a vacuum pump is operated, allowing a material to be concentrated in an evaporator 1 to be evaporated and dehydrated. That is, by the operation of the vacuum pump, an exhaust pipe body 3 is evacuated and also an exhaust branch pipe 5 is evacuated. Therefore, the atmosphere in agitation cylinders 4 is sucked into the exhaust branch pipe 5 and steam fed into a heater body 26 smoothly flows into the agitation cylinders 4. In other wards, new steam is fed into the agitation cylinders 4. When the heater 2 is rotated in this state, new steam is always fed into the agitation cylinders 4. Since the agitation cylinders 4 come into contact with a new concentrate by the rotation of the heater 2 to keep the peripheral surface thereof wetted with moisture, an evaporator heat transfer condition is good and the water content in the material to be concentrated is effectively evaporated.