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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 tothe dust extinction along the line of sight, this event was too faint to bedetected before it reached the peak of magnification. The microlensinglight-curve models indicate a high-magnification event with a maximum of$A_\mathrm{max}\gtrsim200$, very sensitive to planetary deviations. An anomalyin the light curve has been densely observed by the microlensing surveys MOA,KMTNet, and OGLE. From the light-curve modeling, we find a planetary anomalycharacterized by a planet-to-host mass ratio,$q=\left(1.00^{+0.18}_{-0.16}\right)\times 10^{-4}$, at the peak recentlyidentified in the mass-ratio function of microlensing planets. Thus, this eventis interesting to include in future statistical studies about planetdemography. We have explored the possible degeneracies and find two competingplanetary models resulting from the $s\leftrightarrow1/s$ degeneracy. However,because the projected separation is very close to $s=1$, the physicalimplications for the planet for the two solutions are quite similar, except forthe value of $s$. By combining the light-curve parameters with a Galacticmodel, 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-massplanet 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 moreuncertain source distances, two factors that may affect the mass measurementsin the provisional Wide Field Infrared Survey Telescope fields. More events areneeded to investigate the potential trade-off between the higher lensing rateand the difficulty in measuring masses in these low-latitude fields.

OGLE-2015-BLG-1670Lb: A Cold Neptune beyond the Snow Line in the Provisional WFIRST Microlensing Survey Field