iphone is treated by softbank company in japan.
so you can use iphone by paying to softbank in airport.
softbank has many branck in japan and also has a branch in airport ,kansai international airport ,narita airport…many other places.
you will get sim card the branch.
you can use the sim card 105yen/ day.
so if you have iphone ,you wil comfortably use iPhone in japan.