To achieve security in sensor networks, it is important to be able to defend against flooding attack recently considered as an extremely threatening attack. In this paper, we propose a flooding attack detection method as the first defense step, through approximate entropy estimation reflecting resource constraints of sensors. Each detector performs both basic estimation for its own region and overlapped estimation for its own and neighbor regions, against the mobility of attack node. Also, in order to enhance the accuracy of detection even in the various deployments of attack agents, we deploy hierarchically detectors according to network topology. This detector by entropy estimation is simplified by only multiplication calculation instead of logarithm, in addition to providing higher estimation precision of entropy compared to the conventional entropy estimation. Our simulation results indicate that this hierarchical defense is a feasible method, being especially promising for accurate decision through overlapped detection even in frequent handoffs of mobile attack agents.