  We present follow-up spectroscopy on a protocluster candidate selected from
the wide-field imaging of the Hyper SuprimeCam Subaru Strategic Programme. The
target protocluster candidate was identified as a $4.5\sigma$ overdense region
of $g$-dropout galaxies, and the redshifts of $g$-dropout galaxies are
determined by detecting their Ly$\alpha$ emission. Thirteen galaxies, at least,
are found to be clustering in the narrow redshift range of $\Delta z<0.05$ at
$z=3.699$. This is clear evidence of the presence of a protocluster in the
target region. Following the discovery of the protocluster at $z=3.699$, the
physical properties and three-dimensional distribution of its member galaxies
are investigated. Based on spectroscopically-confirmed $g$-dropout galaxies, we
find an overabundance of rest-frame ultraviolet (UV) bright galaxies in the
protocluster. The UV brightest protocluster member turns out to be an active
galactic nucleus, and the other UV brighter members tend to show smaller
Ly$\alpha$ equivalent widths than field counterparts. The member galaxies tend
to densely populate near the centre of the protocluster, but the separation
from the nearest neighbour rather than the distance from the centre of the
protocluster is more tightly correlated to galaxy properties, implying that the
protocluster is still in an early phase of cluster formation and only close
neighbours have a significant impact on the physical properties of protocluster
members. The number density of massive galaxies, selected from an archival
photometric-redshift catalogue, is higher near the centre of the protocluster,
while dusty starburst galaxies are distributed on the outskirts. These
observational results suggest that the protocluster consists of multiple galaxy
populations, whose spatial distributions may hint at the developmental phase of
the galaxy cluster.
