Temporal variations in the P- and S-wave travel times were continuously monitored using ACROSS, which is a newly developed system for utilizing accurately controlled continuous sinusoids for seismic exploration. We made an experiment lasting for 15 months at a site near the Nojima fault which ruptured during the 1995 Kobe earthquake (M7.2). The elastic waves emitted from the ACROSS vibrators located at the surface were observed with seismometers at 800 m and 1700 m depth. The long-term variations of about 2 ms for both the P and S waves were observed during the 15-month experiment. The major cause of these variations is identified to be that of a near-surface property which changes the near-source vibration. The variations in the deeper structures estimated after the reduction of the near-source variation were about 0.5 ms. An S-wave delay of about 1 ms was detected due to the 2000 Western Tottori earthquake (M6.6).