To provide a form capable of efficiently manufacturing a segment of good precision.

A parental form 3 having an almost a triangular pillar shape, whose three faces form arcuate faces 3a being curved in an arcuated state, and three pieces of children forms 4 having arched faces 4a being hinged at its one end in a freely opening and closing manner to the parental form 3 are provided. In a state wherein the children form 4 is closed, the arcuate surfaces 4a hold a specific distance to the arcuate faces 3a of the parental form 3, and a space between those arcuate faces 3a and 4a becomes a cavity 8 for forming a segment. Since three arcuate faces of the parental form mutually play reinforcement roles, three pieces of good precision segments can be simultaneously manufactured.