Disclosed is an RF transmitting device of a mobile radio communication base station system in a CDMA system capable of digital-modulating baseband signals which are converted into band spread signals by using the same path which three frequency assignment frequency signals have for an RF in a multi-carrier mobile radio communication system supporting three frequency assignment, digital-coupling the modulated signals, and up-converting the digital-coupled signal into an IF signal and an RF signal in a sequential order, thereby transmitting the up-converted signal.