To heat works on trays uniformly regardless of mounted positions by heating trays comprising the trays consisted of the trays and heating materials in piles on top of the other or others and arranging plate form heaters around each of the trays.

An electrode piece 11 that can contact the lower surface of a furnace floor 3 is embedded into the center of the upper surface of an elevator base 2 and piled trays 4 are mounted on the furnace floor 3 on the elevator base 2. Each of the trays 4 itself is formed with a carbon heater and the spacers 5 between the trays 4 are formed with conductive material. A carbon heater 9 is arranged to the upper part inside a sintering furnace 1 and the terminals 12 of round bar shape are attached to the both side edge parts downward. Therefore, current supply to the furnace floor 3 is started when the trays 4 are raised to the uppermost position by the elevator 2 and works 7 on each of the trays 4 can be heated uniformly and effectively by heating all of them simultaneously.