To provide a structure of an upper door capable of sufficiently satisfying heat insulating effect, reducing its weight, and properly keeping its durability with respect to a ceramics kiln comprising the upper door for opening and closing an upper portion of a furnace main body.

The upper door 3 is composed of a cover body 3a comprising a polygonal peripheral wall composed of a heat insulating material and formed along a peripheral wall of the furnace main body, holding members 3b having a U-shaped cross-section and holding the cover body in the U-shape in the thickness direction by every side of the polygonal shape of the cover body, a strip member 3c covering side portions of the holding members along the periphery of the cover body, and a fastening member 13 joined to one end of the strip member and fastening and joining the other end of the strip member to reduce a diameter of the strip member.