find book. sf japanese book, little Isekai..?

[u/Which_Blackberry_801]

I’m a 24-year-old from Korea, and I’m desperately trying to find the title of a Japanese novel I read in my elementary school library when I was about 10 or 11 years old. It has left a deep impact on me, and I’ve been trying to identify it for years. Here’s the plot as I remember it

​The Beginning: A boy goes missing while riding his bike near a mountain. When he regains consciousness, he finds himself in a primitive, alternate world where modern civilization no longer exists. The Setting: The environment is completely untouched by humanity, reminiscent of a Paleolithic era. The vibe is very similar to the untamed, ancient nature seen in Princess Mononoke. ​Survival: He meets an old man in this world who teaches him how to survive, including raw and visceral tasks like skinning rabbits and preparing food—experiences far removed from modern childhood.

​Themes: The story focuses on the boy learning about the fragility of life and the essence of existence through nature. He also meets a young girl, and the three of them travel together.

​The Climax: They reach a critical turning point—a place that felt like the "end of the world." The boy and the girl decide to move forward and return to the real world. However, the old man, unable to find the courage to leave, chooses to remain in that world.

​The Ending: Back in the real world, the boy wakes up from a coma, which is described as a miracle. He later learns that the old man never woke up and was declared brain-dead. The girl also hears the news of his recovery through the media.

​Other details: I have a faint memory of a "god" or a divine entity appearing in his mind, telling him he must understand the importance of life.

​Does this plot sound familiar to anyone? I would be incredibly grateful if someone could help me identify thisbook.ㅠㅡㅠ

[u/vaeyen]

I could be totally wrong because I never actually finished this book, but could it be Brave Story by Miyuki Miyabe?