From au website, you could check of the availability for the sim unlocking of au mobile phones and will be able to begin the next step (No processing fee).
Tap「SIM unlocking procedure (My au)」「SIMロック解除の手続き（My au）」
Select the telephone number you wish to process and tap「Select」「選択」
Enter the 4-digit PIN code set up during the initial signing of contract then tap「Next」「次」
Please select either「device purchased at au」「auで購入した端末」or「other devices」「その他の端末」
When「Device purchased at au」「auで購入した端末」is selected, list of devices with purchase history will be displayed.
Devices available for SIM unlocking can be checked.
To proceed with SIM unlocking, check the device you wish to SIM unclock and click「next」「次」
When「other devices」「その他の端末」is selected, please enter IMEI of the device you wish to unclok and click「next」「次」
Tap「reason for unlocking」「解除の理由」,「contact information」「連絡先」,「apply with this detail 」「この内容で申し込む」
For「contact information」「連絡先」,registered email address will appear on the pull down menu.
Please refer to the below list "reason for unlocking".
Completion of temporary registration screen will appear.
By clicking the URL sent to your selected email address, you will be transfered to「SIM unlocking completed screen」「SIMロック解除完了画面」
SIM unlocking has been completed.