Proposal of Neutrino Flavor Oscillation

A brief survey of the history of the neutrino oscillation is presented. Here emphasis is laid on the topics around the first proposal of neutrino flavor oscillation by Nagoya group in 60's. Main contents are presented in my opening address "Birth of Neutrino Oscillation" at the NOW' 98, Amsterdam, hep-ph/9811358, but added some detail of the other topics, especially a first calculation of decay processes $\mu \to e + \gamma$ and $\nu_2 \to \nu_1 + \gamma$ in terms of neutrino flavor mixing, and some arguments relating to the massive neutrino.