To provide a method of molding a thermoplastic base material by which the deterioration such as the occurrence of fogging or bubbles formed on a surface to be functional in a molding base material is prevented not to lose the transparency and the original color exhibited by the molding base material.

The thermoplastic base material 30 is arranged in a mold set having a pair of oppositely arranged molding dies 11, 12 and a sleeve10 into which the dies are inserted and softened by heating the mold set 10 together with the thermoplastic base material 30 and press-molded. In this case, the thermoplastic base material 30 is supported on a part except the surface to be functional so that the surfaces 30a, 30b to be functional are not in contact with functional surface forming surfaces 11a, 11b of a pair of the molding dies 11, 12 and is heated to a temperature at which the thermoplastic base material 30 is softened and deformed by transferring heat from the supported part to the thermoplastic base material.