You need an ETA for Sri Lanka if you have a Japanese passport
Visa options for Sri Lanka
Electronic travel authorization
Travelers need a travel authorization to visit Sri Lanka for Tourism or Business. Visa-exempt visitors entering Sri Lanka must obtain an Electronic Travel Authorization (eTA). Travelers can submit the eTA application online. Once the application is approved, travelers receive their eTA confirmation electronically. An eTA is electronically linked to a passport and valid for up to 30 days. If travelers receive a new passport, they must apply for a new eTA.