To exterminate and remove bacteria involved in laver disease injury in a pollution-free way without strong acid treatment and thus without aggravating marine environment by immersing or sprinkling such bacteria found in laver fisheries in or with a specific kind of natural highly concentrated seawater.

Bacteria involved in laver disease injury and found in laver fisheries are exterminated and removed by immersing or sprinkling such bacteria in or with a kind of natural highly concentrated seawater recycled from the concentrated effluent as a byproduct from a reverse osmosis-type seawater desalination unit 3; alternatively, such bacteria can be exterminated and removed by passing them through a sterile seawater production unit made up of anode 11 and cathode 12 and intended to recycle the wastewater produced in immersing or sprinkling such bacteria in or with a kind of natural highly concentrated seawater from a reverse osmosis-type seawater desalination unit 3.