a. Muroran Institute of Technology, Mizumoto-cho, Muroran, 050-8585 Japan
b. Graduate School of Engineering, Hokkaido University, Kita-ku, Sapporo 060-8628, Japan

Not, only axial train hut also lateral strain were measured during creep tests on Noboribetsu welded tuff under air-dried and water-saturated conditions in which the water contents of specimens were regulated about 0.3% and 17%, respectively. Creep rupture strength under water-saturated condition was smaller than that under air-dried condition. The difference in them was comparable to that in uniaxial compressive strength under both conditions. The axial and lateral strain rates decreased linearly with time in a log-log plot in the primary stage. They increased until a rupture occurs after the strain rates had reached the minimum magnitude. Creep failure time tended to increase as the "decrease rates of the strain rates" in the primary stage increased. Creep failure time and the life after the minimum strain rate increased as the minimum magnitude in the strain rates decreased. In the tertiary stage, the strain rates were inversely proportional to residual life. These relationships were represented by the same curves regardless of the water content. Therefore, the creep failure time would be successfully estimated from the behaviors of either axial or lateral strain rate regardless of the water content by using the experimental results. Thin sections of the specimens that had experienced various levels of the creep deformation were prepared and, observed under a microscope. Nucleation of cracks and pores that distributed along the loading axis in the matrix wereobserved for specimens that had been just before the rupture.