It entered into mining because it acquired a mine as collateral from the loan it had made, and partly because it could buy a mine cheaply from the government, Mitsui then diversified to become the biggest business in pre-war Japan.
Virksomheden gik ind i mineindustrien fordi den havde modtaget en mine som garanti for et lån og delvist fordi den kunne købe en mine billigt af regeringstyret, Mitsui spredte sig således til at blive den største virksomhed i datidens Japan.