Abstract: A base band signal transmitting device in a base station of a mobile communications system is disclosed including a CDMA modem unit constructed in a manner that a plurality of CDMA modems are detachably mounted on one board, each of the CDMA modems converting a digital voice signal into direct band spread signal by sectors, to generate a digital base band signal; a digital signal summing unit constructed in a manner that a digital summing device is mounted on another board, the digital summing device for digital-summing the direct band spread digital signals received from the CDMA modem unit by sectors and transmitting them in parallel; and a signal processing unit including a FIR low pass filter for converting the digital signals received from the digital signal summing unit into parallel signals and removing high frequency component from them, a digital/analog converter for converting the output signals of the FIR low pass filter into analog signals, and a base/intermediate frequency band upward conversion