[en] Radiation protection suits are used at nuclear power plants to protect the personnels from the radioactive materials. The air-tightness of radiation protection suit is one of the most important properties. The leak evaluation method of the suit has not yet been established up to date in Japan. This paper describes the leak determination for radiation protection suits. The main results are as follows; (1) It was found that materials released from the component materials of the protection suit or human body do not disturb the measurement of SF₆ by gaschromatography. (2) The lower determination limit of SF₆ by TCD detector and ECD detector are 40 μg and 2.4 x 10⁶ μg, respectively. (3) Providing 100% SF₆ for out side atmosphere of the suit, 1.2 x 10¹⁰ of the protection factor could be measured by the direct injection method to gaschromatograph, at maximum. (4) The recovery and repeatability of concentration process of SF₆ sample was 103 +- 4%. Using the concentration process, measurable protection factor could be increased up to 2.5 x 10¹². (5) Permeability of the component material of the protection suit was 8 x 10^-8 [ml/cm².min]. Protection factor of the air line-type suit using this material was calculated as 10⁸ order. (author)