To provide a cast part of gray cast iron at a low cost, whose desired surface portion is hardened in as-cast state through a lost pattern casting, and its lost pattern casting method.

Chilling material is coated on a surface portion of a lost pattern made of a foamed resin which corresponds to the portion of the cast part to be hardened, while a facing material is coated on the entire surface of the lost pattern. A cast mold is molded by sand filling in a casting frame with the lost pattern in it and then filled with molten gray cast iron. After solidification, the cast mold is removed to complete a gray cast iron product that has the desired hard portion on its surface.