To easily change an outside diameter of a quartz glass molded product formed by heating and melting a quartz glass preform and molding it.

When melt molding is carried out by using a mold material 10 using a regular polygonal column, which is obtained by assembling carbon plate materials, as an outer cylinder, an outside diameter of a melt molded product to be obtained is changed by changing the width of the plate materials. If the regular polygonal column is a regular octagonal column, when the width of one side in each of the plate materials is changed from a to 1.4a via 1.2a, the diameter (R) of an inscribed circle of the assembled regular polygonal column is increased from 2.4a to 3.4a via 2.9a, so that the diameter can be changed easily.