  We present the analysis result of a gravitational binary-lensing event
OGLE-2005-BLG-018. The light curve of the event is characterized by 2 adjacent
strong features and a single weak feature separated from the strong features.
The light curve exhibits noticeable deviations from the best-fit model based on
standard binary parameters. To explain the deviation, we test models including
various higher-order effects of the motions of the observer, source, and lens.
From this, we find that it is necessary to account for the orbital motion of
the lens in describing the light curve. From modeling of the light curve
considering the parallax effect and Keplerian orbital motion, we are able to
measure not only the physical parameters but also a complete orbital solution
of the lens system. It is found that the event was produced by a binary lens
located in the Galactic bulge with a distance $6.7\pm 0.3$ kpc from the Earth.
The individual lens components with masses $0.9\pm 0.3\ M_\odot$ and $0.5\pm
0.1\ M_\odot$ are separated with a semi-major axis of $a=2.5 \pm 1.0$ AU and
orbiting each other with a period $P=3.1 \pm 1.3$ yr. The event demonstrates
that it is possible to extract detailed information about binary lens systems
from well-resolved lensing light curves.
