Your device must be UNLOCKED.
If you don't know if your device is unlocked or not, please ask the store where you purchased your device.

Your device must be able to use LTE.
VISITOR SIM LTE band
LTE Band 1 (2.1GHz), Band 3 (1.7GHz), Band 19 (800MHz), Band 21 (1.5Hz)
W-CDMA/HSDPA/HSUPA Band 1 (2.1GHz), Band 6 (800MHz), Band 19 (800MHz)

Your device must be safe to use in Japan.

To assure your device is safe to use in Japan, please go to docomo's web site and see if your mobile phone carrier is docomo's roaming partner or not. NOTE: iPhones and iPads have Type Approval marks which assure device's safety that you do not need to check this.

docomo web site (You'll jump to docomo's page. Click the PDF link and find your mobile phone carrier)