In a base station device 12 of an OFDMA mobile communication system, a frequency band storage unit 22 stores multiple frequency bands allocatable to mobile station devices, respectively, in association with predetermined conditions related to communication quality. A communication quality acquisition unit 37 acquires communication quality in communication with each of the mobile station devices. A frequency allocator 24 selects any one of the frequency bands from the frequency band storage unit 22 on the basis of the communication quality acquired by the communication quality acquisition unit 37, and notifies the mobile station device of channel information indicating the selected frequency band. Then, the mobile station device communicates with the base station device 12 in the frequency band indicated by the channel information notified by the frequency allocator 24 in the base station device 12.