A cipher processing device includes: a MAC loss detecting/estimating section which estimates the radio bearer of the lost protocol data unit and a data amount of the lost protocol data unit out of protocol data units produced by division according to the logical channel information included in a packet combining data unit received by a base station of a mobile communication system; an RLC loss detecting/estimating section which detects occurrence of loss in the protocol data units when the protocol data units are put together into a service data unit, and estimates a lost amount of the protocol data units when the protocol data units are put together into the service data unit according to the estimated data amount; and a correction section which corrects a frame number of the service data unit based on the estimated data amount estimated by the detecting/estimating section.