The Brave Blossoms ran in 10 tries to rack up their third bonus-point win of the tournament to leave them on an unassailable 18 from three games ahead of their final match away to the United Arab Emirates on Friday.
Japan raced to a 40-0 half-time lead before their scoring pace slowed in the second period.
"It was a very good performance but we got a little bit tired in the second half," Japan coach Eddie Jones told local media after the final whistle.
Japan opened their campaign with a 121-0 whitewash against the lowly Philippines before overcoming a spirited Hong Kong 38-0 last week.
They have won every edition of the Asian Five Nations and success in next year's tournament will book them a place at the 2015 Rugby World Cup in England.
- Sports & Recreation