A method of detecting abnormality in an unmanned aircraft control system that allows an unmanned aircraft including a first transceiver and a first directional antenna, and a ground facility including a second transceiver and a second directional antenna, to transmit and receive a signal to and from each other, the method including: calculating an inter-antenna distance from the first directional antenna to the second directional antenna; calculating respective transmission performances of the first directional antenna and the second directional antenna; calculating respective reception performances of the first directional antenna and the second directional antenna; estimating a reception level of a radio wave at each of the first transceiver and the second transceiver; and determining occurrence of abnormality in each of the first transceiver and the second transceiver when a difference between the estimated reception level and an actual reception level is equal to or greater than a predetermined value.