I have never looked into the legalities of metal detecting here. I don't know if there are any laws regarding metal detecting, but good judgement should be enough. It simply isn't a popular hobby here from what I can tell. I have never seen anyone else metal detecting nor have I ever seen a sign prohibiting it. No one has ever approached me while metal detecting either, but I stick mainly to the beaches early in the morning. Historic sites could be a different story. The detectorist's code of ethics should serve well. I have read that divers are prohibited from removing anything underwater here and that the government claims ownership to anything in it's territorial waters, but you should check with some local divers to see if that is true/accurate in practice. I am down by Yokosuka, but I have read that the Misawa area has beautiful beaches and fantastic beachcombing (glass floats). Check out the book "I'd Rather be Beachcombing" by Bert and Margie Webber. It provides some good info on beachcombing in the Misawa area.