A radio signal processing apparatus of a portable telephone which can share a first transmit/receive mode such as a digital/analog (D/A) compatible mode and second transmit/receive mode such as a personal communication service (PCS) mode. The radio signal processing apparatus includes: a frequency generator for generating frequencies necessary for a frequency conversion; a first mode converter for converting a frequency of a signal transmitted and received in the first mode according to a prescribed oscillating frequency generated from the frequency generator; a second mode converter for converting a frequency of a signal transmitted and received in the second mode according to a prescribed oscillating frequency generated from the frequency generator; an intermediate frequency processor for converting, transmitting and receiving intermediate frequencies according to a prescribed oscillating frequency generated from the frequency generator; a first switch for selectively connecting an antenna to the first mode converter or the second mode converter; and a second switch for selectively connecting the first mode converter or the second mode converter to the intermediate frequency processor.