Abstract. A chance has two contrary aspects: suddenness as an accidental finding and gradualness as the result of a prepared mind. Such duality of chance discovery resembles the insight process treated by problem solving researches. In this paper, we focus on the insight process in human problem solving, present a broad overview of its suddenness and the gradualness, and introduce our experimental results from the viewpoint of the duality of insight. We believe that our research findings will contribute to studies of chance discovery.