  We report on the environmental dependence of galaxy properties at z=2.15. We
construct multi-band photometric data sets in the (proto-)cluster PKS1138-26
field and in the GOODS field. We then fit spectral energy distributions of the
galaxies with model templates generated with the latest stellar population
synthesis code and derive physical properties of galaxies from the fits. To
quantify the environmental dependence of galaxy properties, a special care is
taken of systematic errors -- we use data sets that have almost the same
wavelength samplings, use the same code to fit SEDs with the same set of
templates, and compare relative differences between the two samples. We find
that the PKS1138 galaxies have similar ages, shorter star formation time
scales, lower star formation rates, and weaker dust extinction compared to the
GOODS galaxies at z~2. This trend is similar to that observed locally,
suggesting that the environmental dependence of galaxy properties is already
partly in place as early as z=2.15. We show that the PKS1138 galaxies assemble
the bulk of their masses ~1 Gyr earlier than field galaxies, i.e., the galaxy
formation depends on environment. Galaxy mergers should frequently occur during
the first collapse of clusters and they might play an important role in driving
the observed environmental dependence of galaxy properties at z=2.15.
