Recently, implantable devices have drawn attention as the near future communication tool. These tools must be wireless when used to communicate with the exterior because they are embedded into the human bodies and pet bodies. So, the study on antennas for implantable devices (implanted antenna) is very important. In addition, the implanted antenna should be small and mechanically robust. In this paper, a cavity slot antenna with a dumbbell shape slot is proposed as an implanted antenna. The model of this antenna operating in the ISM band (2.45 GHz) is analyzed by numerical simulations using the FDTD method. As a result, it is confirmed that the proposed antenna can be used as the implanted antenna