The tectonic stress field was investigated in and around the aftershock area of the Hokkaido Eastern Iburi earthquake (MJMA = 6.7) occurred on 6 September 2018. We deployed 26 temporary seismic stations in the aftershock area for approximately 2 months and located 1785 aftershocks precisely. Among these aftershocks 818 focal mechanism solutions were determined using the first motion polarity of P wave from the temporary observation and the permanent seismic networks of Hokkaido University, Japan Meteorological Agency (JMA), and High Sensitivity Seismograph Network Japan (Hi-net). We found that (1) the reverse faulting and the strike-slip faulting are dominant in the aftershock area, (2) the average azimuths of P- and T-axes are N78° ± 33°E and N3° ± 52°W, respectively, and (3) the average dips of P- and T-axes are 25° ± 16° and 46° ± 20°, respectively: the P-axis is close to be horizontal and the T-axis is close to be vertical. We applied a stress inversion method to the focal mechanism solutions to estimate a stress field in the aftershock area. As a result, we found that the reverse fault type stress field is dominant in the aftershock area. An axis of the maximum principal stress (σ1) has the azimuth of N73° ± 8°E and the dipping eastward of 17° ± 6° and an axis of the medium principal stress (σ2) has the azimuth of N126° ± 91°E and the dipping southward of 16° ± 13°, indicating that both of σ1- and σ2-axes are close to be horizontal. An axis of the minimum principal stress (σ3) has the dipping westward of 64° ± 9° that is close to be vertical. The results strongly suggest that the reverse-fault-type stress field is predominant as an average over the aftershock area which is in the western boundary of the Hidaka Collision Zone. Although the average of the stress ratio is R = 0.6 ± 0.2 in the whole aftershock area, R decreases systematically as the depth is getting deep, which is modeled by a quadratic polynomial of depth.