A transmission rate control method for controlling a transmission rate of uplink user data, based on a received relative transmission rate of the uplink user data which is transmitted via an E-RGCH, includes: transmitting, at a radio base station, any one of Up command which instructs to increase the transmission rate of the uplink user data, Down command which instructs to decrease the transmission rate of the uplink user data, or Don't care command which instructs to maintain the transmission rate of the uplink user data, to the mobile station via the E-RGCH, as the relative transmission rate; and maintaining, at the mobile station, the transmission rate of the uplink user data, when the mobile station receives the Down command or the Don't care command, and when the uplink user data to be transmitted become empty in a transmission buffer of the mobile station.