You may need an embassy visa for Brunei if you have a Japanese passport
Visa options for Brunei
Visitor visa is required for trips longer than 14 days for Tourism
Travelers need a visa to visit Brunei for Tourism for more than 14 days. Travelers do not need a visa if staying for 14 days or less. A paper or embassy visa is the only visa available for travelers visiting Brunei. The visa is typically a stamp or sticker added to the passport. Travelers must obtain this visa from a local embassy, consulate or visa center before traveling.