13.56 MHz band RFID system, which is used for inventory in stores and book inspections in libraries, is a short-distance wireless communication technology that uses a magnetic field. The intensity of the magnetic field used for communication is kept low due to restrictions in the Radio Law, so the communication performance of 13.56 MHz band RFID system is easily affected by the surrounding environment. On the other hand, in tasks such as inventory management and book management, portable RFID reader/writer is required to communicate with tags. In this paper, we assume a portable RFID reader/writer and evaluate the effect of the shape and number of turns of the reader/writer’s antenna coil on the communication performance of the RFID reader/writer. Specifically, we compare the communication distance when a circular coil and a square coil are used as antenna, and show the effect of the coil shape, number of turns, and size on communication performance.