In a current TDMA cellular system, the same frequency channel is reused at separated cells to avoid co-channel interference from other cells. From the view point of the spectrum efficiency, one-cell reuse system where all base stations use the same frequency channel is desirable. However, in such a system, co-channel interference severely degrades BER performance, in addition to the degradation caused by signal distortion due to frequency-selective fading. In this paper, we propose a one-cell reuse TDMA cellular system, where co-channel interference is reduced by adaptively controlling the duration of guard time according to signal to interference and noise ratio (SINR), while the same frequency channel is used at all base-stations. With the proposed system, a current TDMA system is easily extended to one-cell reuse system which achieves higher spectrum efficiency. The system capacity is enhanced with the proposed scheme, while the required qualities such as blocking rate and outage rate are kept.