The World Data Flat is a service provided by au/UQ mobile that allows you to use your smartphone as is in over 160 countries and regions abroad.
If you book in advance in Japan, you can use it for a fixed fee of 490 yen/24 hours (and 690 yen/24 hours in countries and regions not eligible for the Early Discount Campaign). If you do not book in advance, you can use it for a regular fixed fee of 980 yen/24 hours.
You can start using data whenever you want at your destination, and it will automatically stop when the end time comes, so you can use data safely even overseas without worrying about overusing it.
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