It came across that it would be more important to decide what I shouldn't bring than what I should bring.
I thought it good for my body if I had lighter luggae. I had two backpacks, one of them was 25L, the other one was 40L in capacity. 25L one was lighter, but 40L was more comfortable to carry, so I chose the bigger one. It weighted about 2kg by itself though.
At first, I packed clothes, mobile phones and electrical devices. After that I wondered about a sleeping bac. Eventually, I decided to bring a sleeping mat instead of bringing a sleeping bag, because June is the beginning summer in Japna, so it would be wam enought to sleep without a sleeping bag outside. In addition, I planned to stay at Manga cafes where I can borrow blankets for free.
The total weight of my backpack was 8kg, bt I would have to bring a bottle of water every day so the total could be estimated around 9kg. I tartgeted the total weight of my backpack was less than 10kg, so 9kg was OK. In addition, I could throw away clothes if I felt my backpack was too heavy.
Lastly, I seriously concerned about cameras. I had a small camera whose weight is less than 500g. However, I decided not to bring it, because it was annoying to take it out from my backpack when I want to take photos, and hanging it around my neck was also annoying. Anyway, iPhone is one of the best cameras in the world. Therefore, I could take photos by iPhone.
Lastly, it was domestic journey, so I could buy anything if I need at various shops.