Biomimetic robots are possible to investigate aquatic lives without affecting them. Our laboratory has developed a Manta robot that has propulsion mechanism with pectoral fins, which are constructed by mimicking the pectoral fin of the manta ray. However, the swimming speed of the conventional Manta robot does not reach to those of many aquatic lives. In this paper, the propulsion performance of the Manta robot is improved by adopting a 3D streamline shape, which has a less propulsion resistance as the body shape of the robot. In addition, letting the Manta robot be autonomous can overcome a problem that a power cable interrupts the motion of the Manta robot. The effectiveness of the proposed method is proved by several swimming experiments using the improved Manta robot.