  We estimate the abundance of dust in damped Lyman-{\alpha} absorbers (DLAs)
by statistically measuring the excess reddening they induce on their background
quasars. We detect systematic reddening behind DLA consistent with the SMC type
reddening curve, but it is inconsistent with the Milky Way type reddening. We
find that the derived dust-to-gas ratio is, on average, inversely proportional
to the column density of neutral hydrogen, implying that the amount of dust is
constant, irrespective of the column density of hydrogen. It means that the
average metallicity is inversely proportional to the column density of
hydrogen, unless the average dust-to-metal ratio varies with the hydrogen
column density. This indicates that the prime origin of metals seen in DLAs is
not by in situ star formation, with which Z ~ N_{HI}^{0.4} is expected from the
empirical star formation law, contrary to our observation. We interpret the
metals observed in absorbers being deposited dominantly from nearby galaxies by
galactic winds ubiquitous in intergalactic space. When extrapolating the
relation between dust-to-gas ratio and HI column density to lower column
density, we find a value which is consistent with what is observed for Mg II
absorbers.
