  We present results from Subaru/FMOS near-infrared (NIR) spectroscopy of 118
star-forming galaxies at $z\sim1.5$ in the Subaru Deep Field. These galaxies
are selected as [OII]$\lambda$3727 emitters at $z\approx$ 1.47 and 1.62 from
narrow-band imaging. We detect H$\alpha$ emission line in 115 galaxies,
[OIII]$\lambda$5007 emission line in 45 galaxies, and H$\beta$,
[NII]$\lambda$6584, and [SII]$\lambda\lambda$6716,6731 in 13, 16, and 6
galaxies, respectively. Including the [OII] emission line, we use the six
strong nebular emission lines in the individual and composite rest-frame
optical spectra to investigate physical conditions of the interstellar medium
in star-forming galaxies at $z\sim$1.5. We find a tight correlation between
H$\alpha$ and [OII], which suggests that [OII] can be a good star formation
rate (SFR) indicator for galaxies at $z\sim1.5$. The line ratios of
H$\alpha$/[OII] are consistent with those of local galaxies. We also find that
[OII] emitters have strong [OIII] emission lines. The [OIII]/[OII] ratios are
larger than normal star-forming galaxies in the local Universe, suggesting a
higher ionization parameter. Less massive galaxies have larger [OIII]/[OII]
ratios. With evidence that the electron density is consistent with local
galaxies, the high ionization of galaxies at high redshifts may be attributed
to a harder radiation field by a young stellar population and/or an increase in
the number of ionizing photons from each massive star.
