A group robot system includes a plurality of sensing robots and a base station controlling the sensing robots, and establishes communication in a hierarchical manner. The hierarchical structure is formed of a plurality of levels between a plurality of sensing robots with base station as the highest hierarchical level. The first sensing robot detects an object; the second sensing robot conducts further search on the object; and the third sensing robot conducts communication relay between the first sensing robot and the base station. When the first sensing robot detects an object, the base station provides control such that all sensing robots, other than the first, second and third sensing robots, move outside the current area of search.