From the Nobel Foundation:
While at Osaka University, in 1935, he published a paper entitled "On the Interaction of Elementary Particles. I." (Proc. Phys.-Math. Soc. Japan, 17, p. 48), in which he proposed a new field theory of nuclear forces and predicted the existence of the meson. Encouraged by the discovery by American physicists of one type of meson in cosmic rays, in 1937, he devoted himself to the development of the meson theory, on the basis of his original idea. Since 1947 he has been working mainly on the general theory of elementary particles in connection with the concept of the "non-local" field.
Nobel Laureate, Physics Prize, 1949