Provided is foreign-object offshore processing system, including foreign-object inflow prevention devices configured to prevent inflow of foreign object into water intake and provided at interval. Each device includes: collecting pontoon serving as inlet of foreign object and provided to float on sea surface of pool defined by curtain wall, the pool being for drawing cooling sea water from sea toward water intake; bottom net spreading toward sea bottom on upstream side of water intake channel in triangular shape with collecting pontoon as vertex; longitudinal net fixed to two floating chains in suspended manner to spread out in surface area of sea, the two floating chains extending on sea surface from collecting pontoon in correspondence with two sides extending from vertex of triangular shape forming bottom net; and lateral net spreading between bottom net and at least portion of longitudinal net, wherein horizontal net cover covers interval between adjacent devices.