The mobility of Cs in soil decreases with time owing to fixation by micaceous minerals. Such ageing is a critical parameter for estimating and predicting annual change in Cs contamination risk of agricultural products. The decrease in the exchangeable fraction of Cs has traditionally been used as an index of the Cs ageing. Under field conditions, however, exchangeable Cs is influenced by several environmental factors. In this study, we propose a new index to evaluate the Cs ageing with minimum influence of environmental factors. The ratio of the exchangeable Cs fraction to exchangeable fraction of Cs ((Cs/Cs)) eliminates the influence of environmental factors on exchangeable Cs. We assessed the applicability of the (Cs/Cs) index, using a four-year field study of a rice paddy in allophanic Andosol, starting 200 days after the Fukushima Dai-ichi Nuclear Power Plant accident. The influence of K fertilization was also investigated. The Cs and Cs exchangeable fractions varied together, indicating that both were similarly controlled by environmental factors. The values of (Cs/Cs) decreased with time, reflecting Cs fixation by the ageing. The half-time of the (Cs/Cs) decline was 6.6-17.7 years. Relative to K fertilization, the lack of K fertilization seemed to affect the Cs ageing in two ways: the early Cs fixation progressed more rapidly, probably because fewer competing K ions were present, and the long-term ageing process was occasionally hampered, probably by the release of reserve K from micaceous minerals. The (Cs/Cs) values were similar to the ratio of the Cs to Cs transfer factor of the rice straw. Thus, we conclude that the (Cs/Cs) index is reliable for evaluating the Cs ageing, decrease in Cs mobility caused by the diffusion into micaceous mineral interlayer, in the field.