The Subaru FMOS Galaxy Redshift Survey (FastSound). III. The
  mass-metallicity relation and the fundamental metallicity relation at
  $z\sim1.4$
  We present the results from a large near-infrared spectroscopic survey with
Subaru/FMOS (\textit{FastSound}) consisting of $\sim$ 4,000 galaxies at
$z\sim1.4$ with significant H$\alpha$ detection. We measure the gas-phase
metallicity from the [N~{\sc ii}]$\lambda$6583/H$\alpha$ emission line ratio of
the composite spectra in various stellar mass and star-formation rate bins. The
resulting mass-metallicity relation generally agrees with previous studies
obtained in a similar redshift range to that of our sample. No clear dependence
of the mass-metallicity relation with star-formation rate is found. Our result
at $z\sim1.4$ is roughly in agreement with the fundamental metallicity relation
at $z\sim0.1$ with fiber aperture corrected star-formation rate. We detect
significant [S~{\sc ii}]$\lambda\lambda$6716,6731 emission lines from the
composite spectra. The electron density estimated from the [S~{\sc
ii}]$\lambda\lambda$6716,6731 line ratio ranges from 10 -- 500 cm$^{-3}$, which
generally agrees with that of local galaxies. On the other hand, the
distribution of our sample on [N~{\sc ii}]$\lambda$6583/H$\alpha$ vs. [S~{\sc
ii}]$\lambda\lambda$6716,6731/H$\alpha$ is different from that found locally.
We estimate the nitrogen-to-oxygen abundance ratio (N/O) from the N2S2 index,
and find that the N/O in galaxies at $z\sim1.4$ is significantly higher than
the local values at a fixed metallicity and stellar mass. The metallicity at
$z\sim1.4$ recalculated with this N/O enhancement taken into account decreases
by 0.1 -- 0.2 dex. The resulting metallicity is lower than the local
fundamental metallicity relation.
