Nine OBSs (Ocean Bottom Seismographs) were deployed just before the 2003 Tokachi-oki earthquake (M8.0) in the source region, off Hokkaido island, Japan for the period from 7 August to 21 September 2003. The observation area was located about 50-100km landward of the southern Kuril trench where the Pacific plate is subducted toward Hokkaido island. The main shock occurred on 26 September 2003, just several days after this 50-days observation by OBS. Until this big earthquake, the seismic activity in this area has been extremely low in contrast with the adjacent trench areas. No big earthquake has occurred since the 1952 Tokachi-oki earthquake (M8.2). From the phase-reading data, 186 micro- and ultra micro-earthquakes were located in the regular seismic active zones. The b-value of 0.82 from the earthquakes with magnitude from -0.6 to 2.9 could support the fore-shocks of the 2003 Tokachi-oki earthquake. Several interesting phenomena were found in this study as follows. (1) Many clusters of events took place simultaneously in the surrounding source region after a short quiescence period of several days. (2) An enormous number of curious burst signals, which oscillated for only a few seconds, continued to appear at almost OBSs. In particular, at two OBSs DHH and DK being the shortest distance to the Kuril trench, over 3500 bursts of events with amplitudes of 10^-7m/s were counted during the observation period. On the other side, OBS DD, which was close to the epicenter of main event and far away from the trench, around the beginning of September 2003, the rate of occurrence abruptly rose to over twice of the previous rate, which has kept the low rate until then. The start time of activation was almost the same time of the appearance of many widespread clusters of earthquakes. These results will be applied to a research project of the great earthquakes occurring along the trenches.