Multi-carrier (MC) transmission is attractive in next-generation mobile communication systems because of yielding high data rate and robustness against frequency-selective fading. An issue of the MC signal is high peak power. The MC signal is not effective for long distance transmission since longer distance requires higher output power. On the other hand, single-carrier (SC) transmission is effective for keeping a wide coverage area because of low peak power. To obtain both the merits of MC and SC, we consider a hybrid method that uses SC or MC. In this paper, an issue of the SC/MC hybrid system is discussed. We estimate an effect of carrier frequency offset on bit error rate (BER) performance by using computer simulation. The results show that SC is more sensitive to frequency offset than MC. To mitigate effects of the frequency offset, we propose an interference reduction method that removes head and tail symbols of block in the SC transmission. The simulated results show that the proposed method significantly improves the BER performance, and is effective to realize the SC/MC hybrid system.