I may have been carrying it around for half my life, but I have no idea what it opens. I think it might be for a bike lock that I threw away ten years ago because I couldn't find the key. This is contrary to the usual way with keys, which is to throw them away only to discover within a week what it was they opened that you now, for the first time in decades, want to open.
The writing says IOCHIGIY, or IOCHIGIYA if that /- at the end is half an A.
I hope it's not for anything important, now that I've put a scan of it on the Internet from which any thief can make a copy.