  The bipolar nebula, IRAS 19312+1950, is a unique SiO maser source exhibiting
both properties as young and evolved objects. To clarify the nature of this
object, we made molecular line observations with the Nobeyama 45-m radio
telescope. We detected emission from O-bearing (HCO^+, SiO, SO, and SO_2), C-
and N-bearing molecules (CN, CS, HCN, HNC, NH_3, N_2H^+, HC_3N, H_2CS, and
CH_3OH), and their isotopic species (C^17O, ^13C^18O, and C^34S). Line profiles
consist of a weak broad (Dv 30 km/s) and/or a strong narrow (DV< 5 km/s)
components, depending on molecular species. Strong time variations of H_2O and
SiO masers were also observed. Numerical modeling of the envelope with the
LVG-code results in a good fit of the model with a mass loss rate of 2.6 10^-4
M_sun/yr to the observed intensities for the broad-component lines. On the
other hand, non-O-bearing molecules, which have only the narrow profiles, were
found to have abundances typical to those in cool dust clouds. No isotopic
enrichment was found, indicating little evidence of the narrow cool component
being an ejecta of the central AGB star or a possible companion. These facts
compelled us to conclude that IRAS 19312+1950 is an exotic mass-losing evolved
star embedded in a low-mass (~20 M_sun) dark cloud.
