PURPOSE: To improve handling property by a method wherein the elevation of a drainboard, formed in accordance with the inner size of a furnace box, is driven to permit the opening and/or closing of the upper surface of the furnace box as well as the adjustment of the depth of the furnace box upon using.

CONSTITUTION: When respective receiving tools 4 are pushed up to the upper end, a drainboard 3 on the receiving tool 4 is pushed up to a condition that it becomes flush with a floor surface F whereby the upper surface of a furnace box 2 is blockaded and the floor surface F can be used as a flat ordinary guest room. When respective receiving tools 4 are moved downward from this condition, the drainboard 3 is also lowered in the furnace box 2. Accordingly, the height of the drainboard 3, which serves as a foot rest, can be determined by stopping the position in accordance with the preference of an user whereby the user can select a comfortable posture. Further, when the receiving tools 4 are lowered, the drainboard 3 is brought into a condition that it is stored to the lower end of the furnace box 2 whereby foot warmer can be used as an ordinary pit type foot warmer. According to this method, variation between the use of a sitting table type foot warmer and the use of pit type foot warmer can be effected easily whereby the handling property of the foot warmer can be improved.