Abstract: An undersea operation system comprising an undersea operation machine for conducting undersea operations such as undersea construction and the collection of stones and rocks, and a backup ship operatively connected to this undersea operation machine. The undersea operation machine includes a vehicle, arms which are connected to this vehicle, etc. The vehicle and the arms are operated by hydraulic pressure which is supplied through a cable connecting the undersea operation machine to the backup ship, whereby smooth operation of the undersea operation machine is ensured even in a deep sea region.