Non-invasive diagnostics methods are very helpful for cancer diagnosis and they are a research hotspot in the field of biomedicine. Terahertz (THz) photonics is an emerging technology that can be applied in the field of medical diagnostics. This is due to unique features of THz radiation such as harmlessness to biological tissues, strong absorption by water, ability to identify various biomolecules, etc. In this work we have investigated different types of normal and cancer fresh tissues of the stomach using terahertz time domain spectroscopy in reflection mode. Refractive indices of mucous, serous and tumor stomach tissues were obtained in the frequency range of 0,2 - 1 THz. These optical properties are higher for cancer tissue than for mucosa and lower than for serosa. Thus possibility of discrimination of tumor from normal stomach tissue was demonstrated. This study has practical significance for the field of clinical cancer diagnosis and will help to better understand the specifics of the method of pulsed THz spectroscopy applicable to this field.