Recently, remarkable progress in the fields of IT, biotechnology, space-technology has been achieved. However, it seems that only low development has been achieved in the retort sterilization technology, especially in the methods to estimate temperature of central part of foods exactly. An improved estimation method named ATS method (Ambient Temperature Slide method) was developed to solve some theoretical and practical problems in the sterilization technology in 2006. Both accuracy and convenience of the ATS method have been inspected enough so far by many experimental data, however, the ATS method have been doubted regarding propriety of assumption that the deviation rate (β) of temperature of central part of packed foods from space-mean temperature is equal to one (1). This theoretical problem could be solved by using a similarity relation between the temperature history curves of the space-mean and the central part of the packed food. In addition, it was found that a heat-transfer coefficient ( h ) was a very small value compared to conventional expectation by using one-dimensional finite-difference method in retort sterilization. This new finding should be investigated by further experiments.