Satellite/terrestrial integrated mobile communication system (STICS) is an advanced system in which the satellite and terrestrial mobile systems share the frequency band and a mobile terminal can connect both of them. STICS is drawing attention as a robust system in terms of disaster and congestion. However, when the satellite link and the terrestrial link use identical frequencies, the interference is caused by each other and the transmission performances are degraded. Therefore, to ease the interference and to raise the capacity, we propose a new weighting scheme taking the interference into account in the frequency allocation at the transmitter and in the frequency-domain equalization at the receiver of STICS. The numerical results show the system throughput performance of multiple-input multiple-output (MIMO)-based STICS is improved by the proposed scheme.