  Sulfur and zinc are chemically volatile elements, which play significant
roles as depletion-free tracers in studying galactic chemical evolution.
However, regarding red giants having evolved off the main sequence, reliable
abundance determinations of S and Zn seem to be difficult despite that a few
studies have been reported so far. Given this situation, we tried to establish
the abundances of these elements for an extensive sample of 239 field GK giants
(-0.8 < [Fe/H] < +0.2), by applying the spectrum-fitting technique to S I
8694-5, S I 6757, and Zn I 6362 lines and by taking into account the non-LTE
effect. Besides, similar abundance analysis was done for 160 FGK dwarfs to be
used for comparison. The non-LTE corrections for the S and Zn abundances
derived from these lines turned out < 0.1(-0.2) dex for most cases and not very
significant. It revealed that the S I 6757 feature is more reliable as an
abundance indicator than S I 8694-5 for the case of red giants, because the
latter suffers blending of unidentified lines. The finally resulting
[S/Fe]-[Fe/H] and [Zn/Fe]-[Fe/H] relations for GK giants were confirmed to be
in good agreement with those for FGK dwarfs, indicating that S and Zn
abundances of red giants are reliably determinable from the S I 6757 and Zn I
6362 lines. Accordingly, not only main-sequence stars but also evolved red
giant stars are usable for tracing the chemical evolution history of S and Zn
in the regime of disk metallicity by using these lines.
