  We have observed dense gas around the Very Low-Luminosity Ob jects (VeLLOs)
L1521F-IRS and IRAM 04191+1522 in carbon-chain and organic molecular lines with
the Nobeyama 45 m telescope. Towards L1521F-IRS, carbon-chain lines of CH3CCH
(50-40), C4H (17/2-15/2), and C3H2 (212-101) are 1.5 - 3.5 times stronger than
those towards IRAM 04191+1522, and the abundances of the carbon-chain molecules
towards L1521F-IRS are 2 to 5 times higher than those towards IRAM 04191+1522.
Mapping observations of these carbon-chain molecular lines show that in L1521F
the peak positions of these carbon-chain molecular lines are different from
each other and there is no emission peak towards the VeLLO position, while in
IRAM 04191+1522 these carbon-chain lines are as weak as the detection limits
except for the C3H2 line. The observed chemical differentiation between L1521F
and IRAM 04191+1522 suggests that the evolutionary stage of L1521F-IRS is
younger than that of IRAM 04191+1522, consistent with the extent of the
associated outflows seen in the 13CO (1-0) line. The non-detection of the
organic molecular lines of CH3OH (6-2-7-1 E) and CH3CN (60-50) implies that the
warm (~ 100 K) molecular-desorbing region heated by the central protostar is
smaller than ~ 100 AU towards L1521F-IRS and IRAM 04191+1522, suggesting the
young age of these VeLLOs. We propose that the chemical status of surrounding
dense gas can be used to trace the evolutionary stages of VeLLOs.
