A large (M7.1) intraslab earthquake occurred on May 26, 2003, on the upper plane of the double planed deep seismic zone off Miyagi Prefecture. No intraslab earthquakes with magnitudes greater than 7 have occurred since 1926 under the land area of Tohoku, northeastern (NE) Japan. We relocated hypocenters of the 2003 off Miyagi earthquake and its aftershocks to compare the background seismicity prior to this earthquake. We obtained relative earthquake locations using the Double-Difference method [Waldhauser and Ellsworth (2000)] and relative earthquake arrival times measured by waveform cross spectrum. Relocated aftershocks are distributed along a plane steeply dipping to WNW, which coincides with one of nodal planes of the focal mechanism of the main shock. Its dip angle slightly changes near the main shock hypocenter. The hypocenter of the main shock seems to be located near the Moho and aftershocks extend both into the crust and into the mantle of the slab. Many earthquakes occurred near the hypocenter of the main shock before the 2003 event. These events are distributed near the Moho and/or within the mantle of the slab. High background upper-plane seismicity within the mantle may correspond to the area to which the rupture within the crust is easier to extend, causing large intraslab events. We also relocated intraslab earthquakes in the northeastern part of Tohoku. There exist few areas where background upper-plane seismicity extends to the mantle of the slab such as the focal area of the 2003 off Miyagi earthquake. In previous studies, it has been suggested that earthquakes in the upper seismic plane of the double seismic zone in Tohoku are distributed uniformly in space, whereas those in the lower seismic plane are not. However, the results in this study suggest that the spatial distribution of upper plane events is not uniform, either. If intermediate earthquakes are caused by dehydration of hydrated crust minerals, the present result suggests that hydrated minerals are distributed unevenly in the crust.