To estimate long-term deformation and stability of underground structures, it is indispensable to understand time-dependency of rock sample and rock mass. Previous experimental and theoretical studies showed that rock strength was proportional to the 1/(n+1)-th power of loading-rate and creep lifetime was inversely proportional to the n-th power of creep stress, where n was a constant depending on rock type and testing environment. The smaller value of n, the larger time-dependency of rock, which suggests that n can be a useful index of time-dependency. This paper introduced three loading and a creep testing methods to obtain the value of n, and showed their results in uniaxial compression, uniaxial tension and Brazilian tension tests for 38 rocks in the country and overseas. Many rocks showed smaller values of n under water-saturated than air-dried condition. The value of n was proportional to strength under water-saturated condition or low confining pressure, but not under high confining pressure under which failure was more ductile. Rock mass classification systems considering time-dependency of rock were examined as an application of testing data on design or construction of underground structures. An index n was implemented in four classifications, RMR, Q, GSI and RMi, and their proposed equations indicated that rating decreased with an increase in duration of use of structures or in degree of time-dependency of rock mass.