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However, creep tests for rock require appropriate testing environment, long-term maintenance of equipment and high accuracy of strain measurement. Accordingly, the periods of only a few tests were greater than a year in previous studies (Ito & Sasajima 1987, Bérest et al. 2005). Hence important unsolved questions have still remained; under low creep stress, will creep strain be constant? If creep strain will continue to increase, how will it change? Are there any differences between long-and shortterm creep? To obtain knowledge about these issues, a creep testing machine was developed for long-term continuous use. In the autumn of 1994, a uniaxial compression creep test was started with tuff under water-saturated condition. Up to now two sets of creep data have been obtained in two creep tests for over a year (Okubo & Fukui 2002).