Workers have set up barriers at the entrances of five ports on Miyake Island south of Tokyo to prevent drifting pumice stones from entering.
The measure was taken on Saturday, one day after local officials and fishers reported that floating pumice from an underwater volcanic eruption had reached Miyake Island, one of Izu Islands in the Pacific Ocean.
At a port in the southeastern part of the island, divers and fishers have positioned oil barriers to close off the entrance of the port.
A fisherman who usually sails from the port said that he is now unable to leave but the barriers are necessary to keep the pumice away.
He said he is concerned about how the situation will develop.
A subcontractor for the Tokyo Metropolitan Government is patrolling around the port, watching out for the arrival of any stones.
Officials from the Tokyo Metropolitan Government on Saturday collected pumice stones that had washed ashore on a beach in the eastern part of the island.
A local government official also said that he will keep a lookout for any stones.