  The late-time spectra of the kilonova AT 2017gfo associated with GW170817
exhibit a strong emission line feature at $2.1\,{\rm \mu m}$. The line
structure develops with time and there is no apparent blue-shifted absorption
feature in the spectra, suggesting that this emission line feature is produced
by electron collision excitation. We attribute the emission line to a fine
structure line of Tellurium (Te) III, which is one of the most abundant
elements in the second r-process peak. By using a synthetic spectral modeling
including fine structure emission lines with the solar r-process abundance
pattern beyond the first r-process peak, i.e., atomic mass numbers $A\gtrsim
88$, we demonstrate that [Te III] $2.10\,\rm \mu m$ is indeed expected to be
the strongest emission line in the near infrared region. We estimate that the
required mass of Te III is $\sim 10^{-3}M_{\odot}$, corresponding to the merger
ejecta of $0.05M_{\odot}$, which is in agreement with the mass estimated from
the kilonova light curve.
