Cancelling Your Cell Phone Service 

The most convenient method of closing your account is to take the phone to the nearest shop of your phone company. Some companies have offices in Narita, however, it can take quite some time to process the cancellation, so make sure to go several hours before your check-in if you choose this option. 

To cancel your subscription, you will need your phone, plus ID (Alien Registration Card).  You can only cancel in person at a shop, not over the phone. Cell phone companies will not allow someone else to cancel on your behalf, even if instructed to by you.
DO NOT leave Japan without cancelling your phone or you will continue to be charged rental plus possible penalty fees and interest. Most companies bill up to two months behind, so please leave your bank account open for two months after your departure, with sufficient balance to cover two monthsf bills. 

SoftBank 
http://mb.softbank.jp/en/customer_support/ 

Your line will be disconnected as soon as the cancellation procedure is completed. If you are signed up for the Yearly Discount Plan, and you break the contract mid-year, you'll be charged a 42,000 yen cancellation fee. If you are signed up for the Happy Bonus Plan, there will be a 10,500 yen cancellation fee, if you cancel mid-contract. If you have the White Plan, there are no cancellation charges as long as you completed the six month contract period. See their FAQ for more details. 

KDDI au 
http://www.au.kddi.com/english/mobile/ 

The day you apply for cancellation at the au shop will be the cancellation date. 
If you use the following discount services, you may be charged a cancellation fee (depending on the usage period): Annual Discount, Student Discount, Smile Heart Discount, Personal Discount. 

NTT Docomo 
http://www.nttdocomo.co.jp/english/support/procedure/change_release/release/index.html 

If you subscribe to Ichinen Discount, New Ichinen Discount, Fami-wari MAX50, or Hitoridemo Discount 50, set cancellation charges may apply.