To provide a technique which does not cause failure such as warping and cracking of a thin plate-shaped formed product due to heating treatment in a heating treatment method of the thin plate-shaped formed product which is heating-treated in such a state that the thin plate-shaped formed product held between soaking plates is embedded by packing material packed in a heating vessel.

The thin plate-shaped formed product is heating-treated while covering side surfaces of the soaking plates and side surfaces of the thin plate-shaped formed product with surrounding plates. Therein, generation of gaps between the soaking plate and the thin plate-shaped formed product, and between adjoining thin plate-shaped formed products themselves is prevented and, therefore, in-plane temperature distribution of the thin plate-shaped formed products become uniform.

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