Auto flap gate is a water gate which realizes the automatic operation of cutoff and drainage by its buoyancy corresponding to the changes of water levels. The operation states of auto flap gate have been verified by visual obeservation of operator. However, it is difficult for operator to visually observe the operation states and manually operate the gate when it rains very heavily. Therefore, in this paper, a method to systematically judge the operation states of auto flap gate by using observation data of water levels and gate angle is newly proposed. First, a novel classification of eleven operation states is given. Secondly, an algorithm to judge the operation states is proposed, where theoretical gate angle to be utilized in the judgement is calculated based on the moments acting on the gate. In order to evaluate the effectiveness of the proposed method, an experiment by using an actual experimental equipment was performed. By comparing the judgement results based on the proposed method with visual observation, it was verified that the proposed method could successfully achieve the valid evaluation of the operation states.
