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We have performed Atacama Large Millimeter/submillimeter Array (ALMA)observations in $^{12}$CO($J=2-1$), $^{13}$CO($J=2-1$), C$^{18}$O($J=2-1$),$^{12}$CO($J=3-2$), $^{13}$CO($J=3-2$), and CS($J=7-6$) lines toward the activestar-forming region N83C in the Small Magellanic Cloud (SMC), whose metallicityis $\sim$ 1/5 of the Milky Way (MW). The ALMA observations first reveal sub-pcscale molecular structures in $^{12}$CO($J=2-1$) and $^{13}$CO($J=2-1$)emission. We found strong CO peaks associated with young stellar objects (YSOs)identified by the $Spitzer$ Space Telescope, and also found that overallmolecular gas is distributed along the edge of the neighboring HII region. Wederived a gas density of $\sim 10^4$ cm$^{-3}$ in molecular clouds associatedwith YSOs based on the virial mass estimated from $^{12}$CO($J=2-1$) emission.This high gas density is presumably due to the effect of the HII region underthe low-metallicity (accordingly small-dust content) environment in the SMC;far-UV radiation from the HII region can easily penetrate and photo-dissociatethe outer layer of $^{12}$CO molecules in the molecular clouds, and thus onlythe innermost parts of the molecular clouds are observed even in $^{12}$COemission. We obtained the CO-to-H$_2$ conversion factor $X_{\rm CO}$ of $7.5\times 10^{20}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$ in N83C based on virial massesand CO luminosities, which is four times larger than that in the MW, 2 $\times10^{20}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$. We also discuss the difference inthe nature between two high-mass YSOs, each of which is associated with amolecular clump with a mass of about a few $\times 10^3 M_{\odot}$.

ALMA observations of N83C in the early stage of star formation in the Small Magellanic Cloud