In this paper, we analyze a propulsive force generated from pectoral fins for a rajiform-type fish robot from fluid dynamic aspects. A pectoral fin of the rajiform-type fish robot is constructed by multiple fin rays, which move independently, and a film of pushing water. Then, the propulsive force of the fish robot is analyzed from the momentum of the fluid surrounding for every fin between fin rays. The total propulsive force for one pectoral fin is the sum of these momenta. The propulsive speed of a fish robot is determined from the difference of the propulsive force generated from pectoral fins, and the resistance force that the fish robot receives from the water when moving forward. The effectiveness of the proposed method is examined through numerical simulation and actual experimental results.