I rented an (voice + data) iPhone from Global Advanced Communications (GAC) when I visited Japan.

For a 3-week stay, their website states that it will set you back by 14,200 Yen for an iPhone5, or 11,360 Yen for an iPhone4.
We paid for it before arriving in Japan.

Collection of Phone
At Narita Airport, head to the post office to pick up the package using your passport as identification.

Returning of Phone
When collecting the phone, it comes with a pre-paid envelope. Place the phone into the envelope and mail it at Narita Airport post office when leaving the country. Their system was convenient and didn't require any form of face-to-face contact with GAC, which was great especially when departing at odd-hours flight as we could return the phone by mailing it at any time.

Package
The package comes with a factory-settings phone, USB charging cable, adapter for plugging into Japan's power socket and sim-card.

Review
Connectivity was great, we travelled between Tokyo/Hokkaido/Hakone and reception was great throughout. Hope this helps