  In this paper, we report the discovery of a binary composed of a brown dwarf
and a low-mass M dwarf from the observation of the microlensing event
OGLE-2014-BLG-0257. Resolution of the very short-lasting caustic crossing
combined with the detection of subtle continuous deviation in the lensing light
curve induced by the Earth's orbital motion enable us to precisely measure both
the Einstein radius \theta_E and the lens parallax pi_E, which are the two
quantities needed to unambiguously determine the mass and distance to the lens.
It is found that the companion is a substellar brown dwarf with a mass 0.036
+/- 0.005 Msun (37.7 +/- 5.2\ M_J) and it is orbiting an M dwarf with a mass
0.19 +/- 0.02 Msun. The binary is located at a distance 1.25 +/- 0.13 kpc
toward the Galactic bulge and the projected separation between the binary
components is 0.61 +/- 0.07 AU. The separation scaled by the mass of the host
is 3.2 AU/Msun. Under the assumption that separations scale with masses, then,
the discovered brown dwarf is located in the zone of the brown dwarf desert.
With the increasing sample of brown dwarfs existing in various environments,
microlensing will provide a powerful probe of brown dwarfs in the Galaxy.
