A mobile communication system according the present invention including a mobile station UE configured to detect a cell using CC with a measurement target frequency designated by the radio base station eNB, in a measurement gap designated by the radio base station eNB within a predetermined time period TIdentify_Inter, and when the mobile station UE is not performing CA, the predetermined time period TIdentify_Inter is determined based on the number Nfreq of the CCs with the measurement target frequencies, and when the mobile station UE is performing CA, the predetermined time period TIdentify_Inter is determined based on the number Nfreq of the CCs with the measurement target frequencies and the number M of CCs in each of which the measurement gap is set.