This invention relates to a frequency channel assignment system which enables adaptive overlapping use of the same frequency band in a plurality of radio communications systems. The frequency channel assignment system includes radio communications systems 1 and 2 which use a common frequency band, and controllers 31 and 141. The controllers 31 and 141 include a system characteristics information management function 317 configured to manage system characteristics information showing characteristics of frequency channels in the radio communications systems 1 and 2, and a frequency channel assignment function 1411 configured to assign frequency channels to each of the radio communications systems 1 and 2, based on the system characteristics information and channel status information showing status of frequency channels, so as to avoid inter-system interference.