Die capable of being opened, manufacturing method thereof, and mold

a, b, a, b, a, b, a, bIn a die which has a regular polyhedron form and which can be opened into a continuous polygonal form on a plane, boundary areas (Rb) between planes (through ) on the inner side of the regular polyhedron have inclination angles determined according to the regular polyhedron. In addition, engaging portions not exceeding the respective boundary areas (Rb) are placed on the boundary areas (Rb) of predetermined planes (through ). Such a die is formed by injecting plastic resin within a mold according to the form of the opened die. This realizes a three-dimensional die capable of being opened onto a two-dimensional plane, which has only been fictionally conceived, and further provides a manufacturing method thereof.