PURPOSE: To keep antistatic properties and prevent a surface gloss from being impaired even if a synthetic resin sheet material having a conductive layer is threedimensionally processed to an arbitrary shape by thermoforming.

CONSTITUTION: A thermal transfer sheet 2 with a conductive layer 5 having a conductive substance formed on one surface 3a of a releasable polyester film 3 is spread on one surface 7a of a synthetic resin sheet material 1 by a heat treatment. The sheet material 7 is thermoformed with the releasable polyester film 3 attached. The conductive substance of the conductive layer 5 may be made from a conductive tin oxide or a barium sulfate, a carbon black, or the like coated with a conductive tin oxide. These materials are selectively composed in accordance with the purpose. The sheet material 7 is completed as a product by releasing the releasable polyester film 3 after thermoforming.