There are provided an OFDMA communication system capable of reducing a time necessary for a handover without releasing a wireless communication with a base station as a handover source to search base stations, a base station and a mobile station used in the communication system, and a base station switching method.In an OFDMA communication system in which data communication is performed between a plurality of base stations 10 and a plurality of mobile stations, each base station 10 includes subchannel assignment setting unit 15-1 which sets assignment of subchannels including a first subchannel and a second subchannel for each of the mobile stations, the first subchannel including information indicating usable or unusable subchannels of the respective mobile stations and being assigned to the respective mobile stations, the second subchannel including actually-used data. When a request for switching the first subchannel is received from one of the mobile stations 20, the second subchannel is collected and assigned to a same slot as the first subchannel assigned to the one of the mobile stations 20.