In this paper, we propose a system for visualization of radio signal using drone in the field. It is possible to measure the directivity of antennas if we use anechoic chamber. However, it is difficult to obtain the directivity of antennas in the field. In order to get the directivity of antennas in the field, we propose the system which uses detector circuits which convert radio signals to light, receive antennas, and drone. To obtain the directivity in the field, we carried out an experiment. In this experiment, we used Yagi-antennas. The results show that precision of the system is a moderate match to the directivity obtained from the chamber. The results also confirm the proposed system is practical in the field.