This forrest lies on the western side of Miyama in a remote region. This pristine ancient forrest is owned and preserved by Kyoto University and permission and an official guide is required to enter.
You can get an English speaking guide at Tautasha or Kajikaso which are both on this website in places to stay.

There is some wonderful guided hiking here and the plants and wildlife are stunning. You will need to organize well in advance though as it takes time to get the permission.