A band sharing communication system in which there are an N-ary system and an N+1-ary system in descending order of priority of communication, the systems share frequency bands and allocation of a requested band of a terminal station of each system is performed, includes: occupied band setting means for setting an N-ary occupied band and an N+1-ary occupied band adjacent to the N-ary occupied band; band allocation means for allocating a vacant band of the N-ary occupied band with respect to a requested band of an N-ary terminal station and allocating a vacant band of the N+1-ary occupied band with respect to a requested band of an N+1-ary terminal station; and band transferring means for, when there is no vacant band in the N-ary occupied band with respect to the requested band of the N-ary terminal station, if a vacant band equivalent to the requested band is present in the N+1-ary occupied band such that the vacant band is adjacent to the N-ary occupied band, transferring the vacant band from the N+1-ary occupied band to the N-ary occupied band and allocating the vacant band to the N-ary terminal station, and if communication is being performed in a band in the N+1-ary occupied band which is adjacent to the N-ary occupied band and equivalent to the requested band, transferring a vacant band created by making a suspension request to the N+1-ary terminal station performing the communication from the N+1-ary occupied band to the N-ary occupied band and allocating the vacant band to the N-ary terminal station.