You need an ETA for Sri Lanka if you have a Japanese passport
Visa options for Sri Lanka
Electronic travel authorization
Travellers 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). Travellers can submit the eTA application online. Once the application is approved, travellers receive their eTA confirmation electronically. An eTA is electronically linked to a passport and valid for up to 30 days. If travellers receive a new passport, they must apply for a new eTA.