Finally, Japanese phones can now be SIM unlocked without much hassle and without paying a fortune*. The new regulations requiring each carrier to unlock a phone upon request are effective from May 2015. Here are the details.
1. You can only apply for a SIM unlock if your phone is a new model that went into sale after 1st May 2015. If you bought an earlier model after 1st May, you cannot have it unlocked.
* The above does not apply to Docomo phones. All Docomo phones released after the summer of 2010 can be unlocked for a fee (3150 yen in a shop, between 1500 and 2500 yen if purchased online). This applies for models released before 1st May 2015.
2. You can only apply for a SIM unlock - for all carriers, SoftBank and AU included of course - if 180 days have passed since you purchased the phone, or no more than 90 days have passed since you cancelled your contract. As my friend Tokyo_dom neatly put it in programming terms:
X = date of contract start
Y = date of contract end
Z = date of unlock request
If Z > (X + 180) and Z < (Y + 90) then
Also, you need to be the original owner of the phone. Unlocking is free if you do it online from your online subscriber's account on the respective carrier's website. A 3150-yen fee is charged if unlocking is done in a carrier's shop. Details for DoCoMo here (English). Details for SoftBank here (in Japanese). Details for AU here (in Japanese).
3. The following Sharp phones by SoftBank and AU can be SIM unlocked:
* The new unlocking regulations are actually making it more difficult to unlock the latest models if the above conditions (#2) are not met. This has led to the emergence of unofficial unlocking services online - but very expensive ones. As of February/March 2016, unlocking new models after and including the SH-03G can cost about 8000 Yen. Unlock services have also emerged for some older, previosuly not unlockable SoftBank phones, such as the 303SH and 304SH, or the first Aquos Crystal, for between 4500 and 5500 Yen.