Although analytical and experimental studies on the bending propulsion mechanism of fish or cetaceans have been carried out by many researchers, the reasons for high speed and high efflciency of the bending propulsion motion have not been clarified, probably because the dynamic interaction between mechanism and fluid has not been taken into account. In this study, we investigate the characteristics of the force acting on a three-joint bending propulsion mechanism using the analytical method proposed for a coupled system of the mechanism and the fluid. It is found that the inertial force due to the added mass of fluid which produces the forward thrust is dominant when the propulsive displacement in one period of the bending motion is small, and the lift force which produces the drag becomes large when the propulsive displacement becomes large.