To provide a development structure installed in a planar and narrow portion before developed, to be three-dimensionally developed in a planar shape for effectively absorbing impact energy, and to provide an impact absorbing device for developing the development structure arranged at a collision position to absorb impact energy and absorb (relax) impact in the case of unavoidable collision.

The development structure 10 has a planer shape. In the state where a first development member 14 is laid on a second development member 16, when a rotation part 40 of the second development member 16 is thrust at its reverse face side, the rotation part 40 is rotated while being moved and connection beams 42 are deflected with elastoplastic deformation. When the rotation part 40 and a rotation part 20 are moved to the utmost limits in the thrusting direction, the development of the development structure 10 is completed with a ratchet locked to form a solid crossing structure 12 where a plurality of connection beams 22 and the connection beams 42 cross one another.