To prevent an electric heater from moving relatively to an evaporator, by immovably fixing a lengthwise central portion of the electric heater and supporting other portions of the electric heater by means of a support mechanism, which allows movement of the electric heater.

When electricity is passed to an electric heater 6 to raise a temperature of the heater, a central portion of the electric heater 6 is immovably fixed by a support mechanism 10 and so is not moved but other portions of the electric heater 6 are supported by a support mechanism 20, so that they move relatively to an underside of an evaporator 4 and a top surface of a pedestal 21 to freely elongate. When electricity to the electric heater 6 is shut off, both sides of the electric heater 6 move relatively to the support mechanism 20 to freely contract to be returned to its original state. Accordingly, even when passing and shut-off of electricity through the electric heater 6 are repeated, the electric heater 6 will not be moved relative to the evaporator 4.