If you have a Softbank iPhone 6s/7, iPhone 6s/7 Plus, or iPhone SE released after May 2015, you can get it unlocked at a Softbank store (as long as you have owned it for 181 days or more). It can be unlocked from Softbank's website for free, or you can take it into a store and pay 3,000 yen to have them do it.
If you have a Softbank phone released prior to May 2015, it can be unlocked unofficially with a SIM adapter which is fairly cheap. Another method is a "factory unlock" that will cost 20,000 and it could brick your phone, or jailbreak, which could also brick the phone. Info on all of the above below: