SUMMARYThis paper proposes chance index that estimates whether a node is chance in a co‐occurrence network. Recently, chance discovery researches are attractive for several domains. By using chance discovery, we can develop new business or predict earthquake. However, there is a problem that chance discovery requires analysts’ inference from visualized network so that success and failure of chance discovery depend on analysists. In order to solve this problem, we analyzed the features from previous chance discovery researches and build the two hypotheses: (1) chance nodes have high betweenness centrality and (2) chance nodes connect to others with weak links. Based on the hypotheses, chance index is formulated by two terms about betweenness centrality and the strength of links. We confirm the usability of chance index from verification experiments, using Bush network, questionnaire network, interview network, and editorial network.