100,000 yen should be sufficient, excluding your accommodations, transportations and theme park admissions, unless you're expecting to buy an expensive souvenir. I would carry a credit card for peace of mind. 100,000 Yen is roughly 1000USD and it should cover your 12 days.

You can certainly survive in Japan with ¥100,000 (approx. $660 USD), but its "worth" depends entirely on the duration of your stay. For a one-week vacation, ¥100,000 is a very comfortable budget for a solo traveler, covering mid-range accommodation (approx. ¥7,000/night), plenty of high-quality sushi and ramen, and local transport like the subway. However, if you are a resident or student trying to survive for a full month, ¥100,000 is a "starvation" budget. In 2026, average rent in a major city like Tokyo or Osaka for a small studio is between ¥60,000 and ¥80,000, leaving very little for food, utilities, and insurance. For a traveler, this amount would last about 10–12 days if you stay in "capsule hotels" and eat predominantly from convenience stores like 7-Eleven or Lawson. It would not be enough to cover a Japan Rail Pass or long-distance Shinkansen trips. Essentially, ¥100,000 is a great "fun money" amount for a short trip, but as a monthly living allowance in 2026 Japan, it would be extremely difficult without significant subsidies or extremely low-cost housing.