To easily surely prevent the intrusion of the outside air in a waste feeding part to a furnace in a waste treatment furnace (a furnace particularly including a thermal decomposition process).

A substantially vertical throat part is provided in the waste feeding part to the furnace, and level gauges for detecting the position of the waste in the throat part are provided in two upper and lower stages with a predetermined space within the throat part. A waste feeder is controlled so that supplied waste is regularly present between the upper and lower level gauges during the operation of the furnace.