A system achieves seamless localization for a plurality of robots when first some robots moves to a shadow area where GPS signals are not received while remaining second robots receives the GPS signals, by performing an absolute localization for the second robots using the GPS signals; and performing an absolute localization for the second robots using the GPS signals; performing a relative localization for the first robots based on the second robots, thereby determining an absolute location of the first robots. Further, when the second robots move to the shadow area where the first robots have been moved, the system performs the seamless localization by determining a relative location of the second robots based on the first robots on which the relative localization has been performed, thereby determining an absolute location of the second robots based on the relative location of the second robots.