From the precise magnetic field measurements by the Oersted satellite at low altitudes over various local times, a distinct spatial variation at middle and low latitudes was detected in the eastward component residuals (ΔBφ) after the subtraction of a geomagnetic main field model. The average ΔBφ decreases on the dayside as latitude increases, and it increases on the nightside. That is, the latitudinal structure of the ΔBφ has a negative trend in the northward direction on the dayside and a positive trend on the nightside. The slope of the latitudinal profile of ΔBφ is steep on both dayside and nightside under geomagnetically disturbed condition and has a high correlation with geomagnetic activity index such as the ap. These results strongly suggest the existence of a day-night net Birkeland current system that flows into the polar ionosphere on the dayside and flows out on the nightside. These net currents seem to distribute in wide local time range, and the center of the net currents is estimated to be around 1000 LT on the dayside and around 2200 LT on the nightside, which is essentially consistent with previous theoretical predictions and ground geomagnetic observations.