It is known that there is a slow stage of precursory slip prior to a dynamic rupture propagation. Thus it is expected that there should be some physical contrasts between before and after the precursory slip. To detect the contrasts, we performed an observatory experiment in which P- and S-waves were transmitted across a fault in a double direct shear apparatus. Two types of experiments were made: first, Normal Stress Holding Test (NSHT), in which we focused on the effect of stationary contact time on transmission waves. Second, Shear Stress Increasing Test (SSIT), in which we measured local shear stresses in addition to transmission waves. In NSHT we observed increases in amplitude of transmission waves with the logarithm of stationary contact time. The change was several percent after about 3 hours contact. In SSIT we observed a significant increase in amplitude with a shear stress application until a stick-slip occurred. It may be explained by a strengthening of asperity interlocking. However, the detailed mechanism of this phenomenon is uncertain. Further we found a little reduction of increasing rate of amplitude which corresponds to the local precursory slips. This reduction may be attributed to the reset of stationary contact time due to local slips and replacement of asperity contacts with smaller contact sizes. ¿ 1998 American Geophysical Union