PURPOSE: To minimize a gap between a charging port lid and a furnace body when a throwing port is closed by the charging port lid by a method wherein the charging port lid, covering the charging port on the upper surface of the furnace body, is opened by a rail provided on the upper surface of the furnace body so that the lid is moved obliquely upward in parallel to the rim of the charging port with a predetermined angle.

CONSTITUTION: When a charging port lid 20, closing a charging port 16, is supported by rail frames 40, a lower side surface, formed of castable 24, becomes parallel to the upper surface of a furnace body 10 or the upper surface of a rimming frame. The rail frames 40 are provided at both sides pinching the charging port 16 and are supported by horizontal frames 41, extending horizontally from the upper surface of the furnace body 10, and vertical frames 42, extending vertically upward from the horizontal frames 41, with an angle of about 15°. When the charging port lid 20 is opened to charge matters to be incinerated additionally, the charging port lid 20 is elevated along the slope of the rails 31 supported by the rail frame 40. In this case, the charging port lid 20 is released from the charging port 16 upwardly and, at the same time, is moved sidewards whereby the lid is retreated sidewards from the upper surface of the charging port 16.