To provide a method and a mechanism for developing a space structural body in which the space structural bodies are contracted in an overlapping manner when launched in the space, and developed in the space.

A groove 7 is formed around a connection shaft 3, and the groove 7 is connected to a fitting hole 6 therearound. A reflecting mirror has a split structure comprising eight reflecting mirror sections 2. Each section 2 comprises a plate-like reflecting mirror surface 2e formed of a structural material or a film material, peripheral reinforcing members 2a, 2b and 2c, and a fitting shaft 2d. Each fitting shaft 2d is slidably engaged with eight independent grooves 7. When the space structural bodies are launched into the space, eight of them overlap as shown in (a) and (b). Eight sections 2 are moved along the groove 7 during the assembly after the launching, and developed in a circular shape, and peripheral end parts are joined with each other to complete a circular reflecting mirror.