By mooring or slowly moving the catamaran in a current through which a garage patch formed of disused plastic particles over than 90% is drifting, a heap of the marine debris flows into the bow of the catamaran and then is smashed. While the disused plastic particles that have been smashed at the bow of the catamaran is moving from the bow toward the stern by way of the tunnel between hulls of the catamaran, the plastic particles are first classified by differences of densities. These first classified plastic particles are collected and then transferred into a narrow and long inside of the catamaran, being finally classified by a triboelectrostatic action. Thereafter, these finally classified plastic particles are made to be pallets which are to be transported to a land facility for recycling or abandoning them.