Understanding of the physics under shock compression has been an important subject over the past century. Many experimental results have revealed that the shock-compressed material initially behaves as a purely elastic medium, and finally results in plastic deformation. However, the details of the transition between these two states has not been fully understood. We directly observed transient structural dynamics of iron induced by shock wave using brilliant X-ray Free Electron Laser (XFEL) pulses with a duration less than 10 femtoseconds. We found that the shock at an early stage produces an elastic strain corresponding to a stress of 22 GPa. We suggest that the giant elasticity induces generation of dislocations causing a high density of lattice defects inside the material, and that the plastic deformation without dislocations occurs, which was theoretically predicted but has not been observed. Our findings elucidate mechanisms of ultrafast phenomena under extreme conditions driven by shock compression, and promote to emerge novel characteristics in matters.