In a radio base station (100) that communicates with a mobile station according to the PHS standards, a reception timing detection unit (55) measures the time when a UW signal of a signal separated by a signal regulating unit (51) for each mobile station is received with reference to the starting point of the time slot of a TDMA/TDD frame, as a reception timing signal. A control unit (80), when assigning a communication channel according to spatial multiplexing to a mobile station, allocates a communication channel in which a reception timing difference between the new mobile station and all mobile stations communicating is equal to or exceeds a threshold value.