1. you dont have access to Wifi network
2. you want to check your softbank mail
3. you dont want to pay for the packets
you can simply go to www.mail2web.com give your softbank mail address(
If you are not comfortable providing this detail to the site, there is yet another way to access your mail.
Configure your PC`s mail client with the softbank`s address and password. I tried with my Mail application of Mac OS X and it worked without a glitch. I am sure that it will work with other mail clients like Microsoft Outlook, Thunderbird etc. Softbank uses IMAP (not POP3) as the incoming mail server(imap.softbank.jp). Hence, you cannot add your softbank account to Gmail. Gmail only allows POP3 server accounts to be added.