A radio base station apparatus in a communication system in which best effort type transmission is carried out between the radio base station apparatus and mobile terminal apparatus through a wireless channel, capable of reducing a wait time in a buffer. The radio base station apparatus (300) is provided with a determining section (308) that determines a transfer rate of data transferred from a radio network control apparatus (500), a storage section (304) that temporarily stores data transferred from the radio network control apparatus at the transfer rate, a transmission section (301) that transmits the data stored in the storage section to a mobile terminal apparatus by radio, a wait time measuring section (306) that measures a wait time of data in the storage section and a transmission rate calculation section (307) that calculates an average transmission rate of data transmitted by radio to the mobile terminal apparatus, wherein the determining section (308) regards a value obtained by multiplying the average transmission rate by a coefficient according to the wait time as the transfer rate.