This is not precisely true. It is legal to carry a non-locking knife with a blade less than 60 mm. The typical Victorinox Classic SD size is legal to carry. A carried knife must be concealed--you cannot, for instance, hang a Classic SD on your keychain dangling from your belt or handbag, but you can have it in your pocket.

Otherwise, you have to have a definite purpose for transporting any knife outside your home. Camping qualifies (that is to say *actually going camping*, just like in the UK, not carrying it around "just in case you decide to go camping"), and there are exceptions for tool kits, cooking equipment, and fishing equipment. However, I would highly recommend not pushing your luck, as you definitely want to avoid negative interactions with police in Japan.

Even inside your own home, you can't have a non-cooking knife with a blade longer than 150 mm without a special permit from the prefectural public safety commission.