PURPOSE: To excellently heat the inside of a chamber, by decreasing the time variation of room temperature by continuously controlling an electric heating device while mounting a sensor for detecting temperature at locations positioned not more than 50cm on the floor, where are not directly subject to the influence of radiant heat air currents.

CONSTITUTION: An alternating current power source 11 is connected to an electric heating device 13 through a phase controller 12. An output terminal of a sensor 14 for detecting temperature for detecting room temperature is guided to a phase controller 15 for generating control signals while responding to the sensor 14, and the output terminal is connected to a control terminal of the phase controller 12. The electric heating device 13 is disposed to one side lower portion of this chamber, and the sensor 14 detecting temperature is arrnaged at locations in an extent positioned not more than 50cm on the floor, where is not directly subject to the influence of the radiant heat air currents of the heating device 14.