We present results from Subaru/FMOS near-infrared (NIR) spectroscopy of 118 star-forming galaxies at $zsim1.5$ in the Subaru Deep Field. These galaxies are selected as [OII]$lambda$3727 emitters at $zapprox$ 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$6584, and [SII]$lambdalambda$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 $zsim$1.5. We find a tight correlation between H$alpha$ and [OII], which suggests that [OII] can be a good star formation rate (SFR) indicator for galaxies at $zsim1.5$. 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.
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