  We present the analysis of the microlensing event OGLE-2015-BLG-1670,
detected in a high-extinction field, very close to the Galactic plane. Due to
the dust extinction along the line of sight, this event was too faint to be
detected before it reached the peak of magnification. The microlensing
light-curve models indicate a high-magnification event with a maximum of
$A_\mathrm{max}\gtrsim200$, very sensitive to planetary deviations. An anomaly
in the light curve has been densely observed by the microlensing surveys MOA,
KMTNet, and OGLE. From the light-curve modeling, we find a planetary anomaly
characterized by a planet-to-host mass ratio,
$q=\left(1.00^{+0.18}_{-0.16}\right)\times 10^{-4}$, at the peak recently
identified in the mass-ratio function of microlensing planets. Thus, this event
is interesting to include in future statistical studies about planet
demography. We have explored the possible degeneracies and find two competing
planetary models resulting from the $s\leftrightarrow1/s$ degeneracy. However,
because the projected separation is very close to $s=1$, the physical
implications for the planet for the two solutions are quite similar, except for
the value of $s$. By combining the light-curve parameters with a Galactic
model, we have estimated the planet mass
$M_2=17.9^{+9.6}_{-8.8}\,\mathrm{M}_\oplus$ and the lens distance
$D_\mathrm{L}=6.7^{+1.0}_{-1.3}\,\mathrm{kpc}$, corresponding to a Neptune-mass
planet close to the Galactic bulge. Such events with a low absolute latitude
($|b|\approx 1.1\,\mathrm{deg}$) are subject to both high extinction and more
uncertain source distances, two factors that may affect the mass measurements
in the provisional Wide Field Infrared Survey Telescope fields. More events are
needed to investigate the potential trade-off between the higher lensing rate
and the difficulty in measuring masses in these low-latitude fields.
