Japanese police are often anal about things like this. Even beach rings. I have been approached by police a couple times in regards to turning in finds (once on beach and once in a park), and had the police called on me by curious onlookers. The time they were called on me in a park they hit me with the "turn in finds of value" as well as the "destruction of public property" for cutting roots to nearby bushes. Had to open up a couple plugs to show them I was digging around roots not through them to avoid a $1,000 ticket and confiscation of my detector.So to apply this to Japan's L&F laws : Is it actually applied to fumble fingers rings on the beach ? I dunno. The guy I talked to said that it didn't apply to beach jewelry . Or, at least .... no one said squat to him. Does this mean that if he had gone asking enough bureaucrats there "Can I ?", that he might have been appraised otherwise ? Sure. And then he'd have just become the latest victim of : "No one cared TILL you asked"
As far as I've been able to research, however, there are no specific laws about metal detecting, and the Lost and Found laws "appear" to be only for items found on the surface.
Metal detecting is almost unheard of in Japan as a hobby, and 'treasure hunting' (as they see it) is generally frowned upon. Add that to being a white foreigner that is 'scary' to approach, overly interested people will not hesitate to call the police on you.