  We performed 12CO(J=1-0) (hereafter, CO) observations towards 12 normal
star-forming galaxies with stellar mass of Mstar=10^10.6-10^11.3 Msun at
z=0.1-0.2 with the 45-m telescope at the Nobeyama Radio Observatory (NRO). The
samples are selected with Dn(4000) that is a strength of the 4000 \AA break,
instead of commonly used far-infrared (FIR) flux. We successfully detect the CO
emissions from eight galaxies with signal-to-noise ratio (S/N) larger than
three, demonstrating the effectiveness of the Dn(4000)-based sample selection.
For the first time, we find a tight anti-correlation between Dn(4000) and
molecular gas fraction (fmol) using literature data of nearby galaxies in which
the galaxies with more fuel for star formation have younger stellar
populations. We find that our CO-detected galaxies at z~0.1-0.2 also follow the
same relation of nearby galaxies. This implies that the galaxies evolve along
this Dn(4000)-fmol relation, and that Dn(4000) seems to be used as a proxy for
fmol which requires many time-consuming observations. Based on the comparison
with the model calculation with a population synthesis code, we find that star
formation from metal enriched gas and its quenching in the early time are
necessary to reproduce galaxies with large Dn(4000) and non-zero gas fraction.
