My presentation consists of three sections.

§1 Ishiwara's original contributions to early quantum theory

Jun Ishiwara (1881-1947) was one of the earliest in the world and only one Japanese that contributed to early quantum theory. In 1911, he submitted his own original paper in German, ‘Contribution to light-quantum theory’, in which he deduced Planck's radiation formula from the viewpoint of radiation as a collection of light-quanta, to some extent, in a similar way to Bose's statistics of 1924. Also in that paper, Ishiwara tried to explain wave-like behaviour of radiation from that viewpoint, similarly to what de Broglie did by way of the phase wave in 1923. In 1915, Ishiwara published an original paper in German, ‘The universal meaning of the quantum of action’. He criticized Bohr's quantum condition as that the scaler quantity h could not be the fundamental meaning of angular momentum, which is a vector quantity. Instead he extended Planck’s quantum condition for systems with one degree of freedom that their phase space should be divided into equally probable regions of area ∫∫dqdp=h. Thus, he introduced his own quantum condition applicable to systems with many degrees of freedom. His 1915 paper, along with W. Wilson's paper of 1915, presented for the first time in the world, the quantum condition rendering Planck's radiation formula and Bohr's theory of atomic constitution at the same time. The difference between Ishiwara's and Wilson-Sommerfeld's quantum conditions is whether the symbol Σ, showing the summation over degrees of freedom, is put in front of the integral symbols or not. In 1917, Einstein criticized that Wilson-Sommerfeld's quantum condition, which lacked it, is not independent of the choice of coordinates.

§2 Reception of Early Quantum Theory in Japan

The reception of early quantum theory in Japan started in 1911, which was the year of the first Solvay conference on the subject of "Theory of radiation and quantum". Thus, it was not so much later than that in Europe. Nevertheless, compared with the reception of relativity in Japan, that of quantum theory was modest and limited to the world of specialists.

§3 Reception of Quantum Mechanics in Japan

In 1926, six young Japanese physicists started a new meeting for reading physics papers in turn. In the next year, they published Summary of Physics Papers Vol.1, which included expositions in Japanese of those papers by de Broglie, Schrödinger, Heisenberg, Born and Jordan, Dirac, et al. Also in 1927, a new journal in Japanese, Journal of Mathematico-Physical Society of Japan started, on every issue of which were printed abstracts of important papers, including those on newly built quantum mechanics. In December 1928, Sommerfeld visited to Japan, and made lectures on quantum mechanics. Late in that month, Yoshio Nishina returned home, who had been studying at Copenhagen under Bohr for six years, and derived famous Klein-Nishina formula for Compton scattering. Centering on Nishina's laboratory, research using quantum mechanics flourished in Japan.