  The high-resolution ($R\sim48\,000$) optical spectrum of the B-type
supergiant LS 5112, identified as the optical counterpart of the post-AGB
candidate IRAS 18379-1707, is analysed. We report the detailed identifications
of the observed absorption and emission features in the wavelength range
3700-9200 A for the first time. The absorption line spectrum has been analysed
using non-LTE model atmosphere techniques to determine stellar atmospheric
parameters and chemical composition. We estimate $T_\text{eff}=18~000\pm1000$
K, $\log g=2.25\pm0.08$, $\xi_{\rm t}=10\pm4$ km/s and $v \sin i=37\pm6$ km/s,
and the derived abundances indicate a metal-deficient ([M/H]$\approx-0.6)$
post-AGB star. Chemical abundances of eight different elements were obtained.
The estimates of the CNO abundances in IRAS 18379-1707 indicate that these
elements are overabundant with [(C+N+O)/S]=+0.5$\pm$0.2 suggesting that the
products of helium burning have been brought to the surface as a result of
third dredge-up on the AGB. From the absorption lines, we derived heliocentric
radial velocity of $V_\text{r}=-124.0\pm0.4$ km/s. We have identified permitted
emission lines of O I, N I, Na I, S II, Si II, C II, Mg II and Fe III. The
nebula forbidden lines of [N I], [O I], [Fe II], [N II], [S II], [Ni II] and
[Cr II] have also been identified. The Balmer lines H$\alpha$, H$\beta$ and
H$\gamma$ show P-Cygni behaviour clearly indicating post-AGB mass-loss process
in the object with the wind velocity up to 170 km/s.
