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Systematic studies of cosmic rays in Japan were started when the Nishina Laboratory was established in Riken in 1931, after Nishina came back from the Niels Bohr Institute in Copenhagen. He was already known as an author of Klein-Nishina formula. He spent a great deal of effort to spread modern physics in Japan, inviting Dirac and Heisenberg to give series of lectures at the university of Tokyo, which later established the strong group of theoretical physicists including Yukawa, Tomonaga, Sakata and others. He also spent a great deal of effort in comprehensive studies of cosmic-rays including the precise measurements of the mass of mesons, of the intensity at the deepest underground of 3000 m.w.e., and of other research to be discussed. It is to be noted that the encouragement by the theoretical group of Yukawa, Tomonaga and others stimulated the recovery of the research after World War II in Japan. [ABSTRACT FROM AUTHOR]