To shorten the manufacturing days of a mold by a method wherein a clay layer and a first mold frame are laminated on a wooden die of a molded form and clay of the clay layer is taken out of a hole, formed on the first mold frame, while an uncured rubber composite is filled and cured instead of the clay to form the first elastic mold and, subsequently, a second mold is formed in the same manner so as to position the first mold frame at the lower part thereof.

The wooden die 4 of a molded form is put on a setter 2 in a wooden frame 1 and a clay layer 5 is formed on the surface of the wooden die 4. FRP 6 is laminated on the surface of the clay layer 5 to form a first mold frame 7. A hole 8 is bored on the first mold frame 7 to take the clay of the clay layer 5 out of the hole 8 and form a gap between the wooden die 4 and the first mold frame 7. An uncured rubber composite 9 is poured into the gap through the hole 8 and is cured to form the bottom force 11 of a split mold. The bottom force 11 and the wooden die 4 are turned up-and-down integrally and the same treatment is applied on the exposed surface of the turned wooden die 4 to form the top force 16 of the split mold. According to this method, the split mold, consisting of the top force 16 and the bottom force 11, can be manufactured in short days.