  In 1943 from September to December Kiyoshi Oka wrote a series of papers
numbered from VII to XI, as the research reports to Teiji Takagi (then,
Professor of Tokyo Imperial University), in which he solved affirmatively the
so-called Levi Problem (Hartogs' Inverse Problem termed by Oka) for unramified
Riemann domains over $\C^n$. This problem which had been left open for more
than thirty years then, was the last one of the Three Big Problems summarized
by Behnke--Thullen 1934. The papers were hand-written in Japanese, consist of
pp.~108 in total, and have not been published by themselves.
  The method of the proof is quite similar to that of Oka IX published in 1953
except for the use of Coherence Theorems: There, in the unpublished papers
1943, he proved some ideal theoretic properties of holomorphic functions, which
was sufficient to prove the J\^oku-Ik\^o (lifting principle) with estimates;
then it led to the solution of the Levi (Hartogs' Inverse) Problem. In this
series of papers, he already had in mind a project not only to settle the Levi
(Hartogs' Inverse) Problem of general dimension, but also to deal with the
problem for ramified Riemann domains.
  Reading the series of unpublished papers 1943 we see the dawn of the then
unknown notion of "Coherence" or {\em "Id\'eaux de domaines ind\'etermin\'es}"
in Oka's terms, and may observe that the turn of years "{\em 1943/'44}\," was
indeed the watershed in the study of analytic function theory of several
variables.
  At the end of this note we will discuss a problem which K. Oka left and is
still open.
