Samples of fresh excised tissues obtained from patients who had undergone gastric cancer have been investigated. Samples consisted of cancer zone, normal zone and pathologically changed zone. Their optical properties and spectral features were investigated by terahertz time-domain spectroscopy (THz-TDS) in reflection mode. It was found that waveforms of reflected signals from normal and cancer tissues and their optical properties were well distinguished, so it can be concluded that it is possible to discriminate gastric cancer tissue from normal by using THz-TDS.