  (Abridged) We present a discovery of definitive large-scale structures around
RXJ0152.7--1352 at z=0.83 based on spectroscopic redshifts. In our previous
papers, we reported a photometric identification of the large-scale structures
at z~0.8. A spectroscopic follow-up observation was carried out on 8 selected
regions covering the most prominent structures to confirm their association to
the main cluster. In six out of the eight fields, a well isolated peak is
identified in the distribution of spectroscopic redshifts at or near the
cluster redshift. This is strong evidence for the presence of large-scale
structures associated to the main cluster at z=0.83. We then investigate
stellar populations of galaxies in the structures. The composite spectra are
constructed from a number of red member galaxies on the colour-magnitude
sequence. We consider three representative environments -- cluster, group, and
field. The ``cluster'' red galaxies do not show any sign of on-going or recent
star formation activities and the passive evolution can naturally link them to
the present-day red sequence galaxies in the Sloan Digital Sky Survey. In
contrast, the red galaxies in ``groups'' and in the ``field'' tend to show
signs of remaining and/or recent star formation activities characterized by
weak [OII] emissions and/or strong H delta absorptions. Our current data seem
to favour a scenario that star formation is truncated in a short time scale
(<1Gyr). This would imply that galaxy-galaxy interactions are responsible for
the truncation of star formation.
