To further efficiently convert energy provided by propellers of a biaxial ship into propulsion by recovering energy of a swirling flow.

A propulsion device of the biaxial ship includes a pair of propellers 5-1 and 5-2 rotating externally, a rudder 6 arranged in the rear of the propellers, and a fin 8 installed in the rubber and having an elevation angle of rising toward the stern side. The propulsion is generated when the upward swirling flow generated in the center line vicinity by the pair of propellers strikes on the fin, and propulsion efficiency is improved.