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This paper proposes a hierarchical formation control using a target tracking control law based on the vector field method such that a decentralized and flexible formation control can be achieved without additional motion planning. Previously, many researchers have dealt with the control laws for the rigid formation, where the line of sight toward the leader is controlled for the leader-follower formation control. However, a width change or a collision of the formation can occur since a limited motion of the rigid formation can occur when the formation control maintains the line of sight. Therefore, the formation of multiple mobile robots is required to be flexible, keeping the width and curvature of the formation. To this end, a formation control law based on a vector field method is proposed, and a hierarchical formation structure is introduced in such a way that it consists of a line formation and a column formation based on the leader-follower formation strategy. First, a subgroup, which consists of several robots, is generated using the line formation, and then, the overall formation structure is constructed from several subgroups using the column formation. Finally, we show the stability of the whole formation. The stability analysis and simulation results of the proposed hierarchical formation control using this vector field method are included to demonstrate the practical applicability of the proposed method.