  We present deep near-infrared (NIR) imaging of Sh 2-209 (S209), a
low-metallicity (${\rm [O/H]} = - 0.5$ dex) HII region in the Galaxy. From the
NIR images, combined with astrometric data from Gaia EDR3, we estimate the
distance to S209 to be 2.5 kpc. This is close enough to enable us to resolve
cluster members clearly ($\simeq$1000 AU separation) down to a mass-detection
limit of $\simeq$0.1 $M_\odot$, and we have identified two star-forming
clusters in S209, with individual cluster scales $\sim$1 pc. We employ a set of
model luminosity functions to derive the underlying initial mass functions
(IMFs) and ages for both clusters. The IMFs we obtained for both clusters
exhibit slightly flat high-mass slopes ($\Gamma \simeq -1.0$) compared to the
Salpeter IMF ($\Gamma = -1.35$), and their break mass of $\simeq$0.1 $M_\odot$
is lower than those generally seen in the solar neighborhood ($\sim$0.3
$M_\odot$). In particular, because the S209 main cluster is a star-forming
cluster with a larger number of members ($\sim$1500) than the number
($\sim$100) in regions previously studied in such environments, it is possible
for the first time to derive the IMF in a low-metallicity environment with high
accuracy over the wide mass range 0.1--20 $M_\odot$.
