[en] The present works are concerned with the thermal degradation or loss of weight of the mixtures contained in vessels. The resins used is an epoxis, 'NS-4-FR' and the fire resisting material mixed is aluminum hydroxide. Firstly, the effect of temperature on degradation was surveyed on the NS-4-FR by continuous heating tests at different temperatures under unenclosed, or open conditions; 125degC, 150degC, 175degC and 200degC. Then, the effect of difference and shape of materials was surveyed at 150degC. Secondly, continuous heating tests were conducted under open and enclosed conditions at 125degC, 150degC and 175degC and the effect of enclosing the mixtures was obtained. Finally, continuous and cyclic heating tests were conducted under enclosed conditions at 125degC, 150degC and 175degC, which was followed by long-term cyclic heating tests at 150degC for more than one year. As the results, following emperical equation was obtained for the thermal degradation at 150degC of the NS-4-FR: W = 0.55 logD - 0.41 where, W: thermal degradation (weight loss), %, D: heating period, day. From this equation, weight loss in 20 years is obtained as less than 2% which is insignificantly small for shielding effect. (J.P.N.)