A vertical annealing furnace structure is provided to prevent overheating of a muffle by preventing flames ejected from burners from being concentrated on local positions on a muffle and preventing partial heating of a part of the muffle adjacent to the burners due to a temperature deviation between the part of the muffle and other parts of the muffle. In an ordinary vertical annealing furnace for heat-treating a steel sheet in a reducing hydrogen atmosphere, comprising: a furnace body(20) with an exhaust port(22) and a plurality of burners(40); and a muffle(30), a vertical annealing furnace structure for preventing overheating of the muffle is characterized in that: the burners are mounted on the furnace body such that directions of flames generated from the burners are essentially identical to a tangential line direction formed by an outer diameter part of the muffle; and the exhaust port is formed in such a manner that a longitudinal direction of a part of the exhaust port connected to the furnace body is essentially equal to flow directions of flames and combustion gas that are generated from the burners and moved along the circumferential direction of the muffle.
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