A communication system includes at least one base station and mobile stations, wherein the base station comprises: a transmission section for transmitting a high-speed packet data (HSDPA) to the mobile stations in an channel for transferring the HSDPA of a downlink; a receiving section for receiving quality information QI about the downlink transmitted from the mobile stations based on a transmission cycle notified by the mobile stations, wherein the transmission section notifies the transmission cycle to the mobile stations, and selects the transmission cycle from a group consisting of 0, 1 and at least two positive integers without having a relation of a multiple, and of at least zero positive integer greater than the at least two positive integers without having the relation of a multiple, the receiving section comprises a table for identifying a pre-defined modulation mode and a transmission speed value with respect to the downlink quality, and receives the value as the quality information transmitted from the mobile stations based on the transmission cycle. The value is obtained from a table sharing with the table based on the downlink quality information by the mobile stations.