The heritage conservation act (in swedish:
http://www.riksdagen.se/Webbnav/index.aspx?nid=3911&bet=1988:950 ) is the relevant legislation in this case (chapt 2 §§18-20). Very briefly the legislation says:
1/ metal detectors may only be used with the premition of the County Administrative Board (the permit is hard to get)
2/ archaeological sites may not be detected, if a site is unvieled during metal detecting the activity should stop and authotities called upon. You may not even carry a metal detector over a known archaeological site!
The application of the legislation is different for archaeologists involved in excavations (which of course also require permits), the military etc.
The maximum penalty for violation of the law is inprisonment for a period of 6 months (usually 2 months in the cases I have studied concerning metal detectors)