The present invention provides a radio base station using the conventional downlink transmit diversity which can compensate for the degradation of transmission quality even when a plurality of users (mobile terminals) share a single time slot using a code division multiple system. The radio base station includes a channel estimator 16 for performing channel estimation of an uplink time slot; a transmit diversity controller 14.sub.1 to 14.sub.N for controlling the transmit diversity used for downlink time slots DL#1 to DL#N based on said channel estimation and the number of users sharing the downlink time slot when using a code division multiple system; a data transmitter 15 for transmitting data via downlink time slots DL#1 to DL#N using the controlled transmit diversity.