At present, people say that the data I was collecting with our hand-made analog computer using vacuum-tubes on the 27th of November, 1961 is the oldest example of chaos discovered in a second-order non-autonomous periodic equation expressing behavior of a periodically forced negative resistance oscillator.In this talk, I would like to reflect upon the motive and circumstances of my research and difficult technical problems to solve.The talk may inspire especially young researchers, who will engage in Nonlinear Dynamics Research from now on.