  We present statistics of 133 faint 1.2-mm continuum sources detected in about
120 deep ALMA pointing data that include all the archival deep data available
by 2015 June. We derive number counts of 1.2 mm continuum sources down to 0.02
mJy partly with the assistance of gravitational lensing, and find that the
total integrated 1.2 mm flux of the securely identified sources is
22.9^(+6.7)_(-5.6) Jy deg^(-2) that corresponds to 104^(+31)_(-25) % of the
extragalactic background light (EBL) measured by COBE observations. These
results suggest that the major 1.2 mm EBL contributors are sources with 0.02
mJy, and that very faint 1.2 mm sources with ~< 0.02 mJy contribute negligibly
to the EBL with the possible flattening and/or truncation of number counts in
this very faint flux regime. To understand the physical origin of our faint
ALMA sources, we measure the galaxy bias bg by the counts-in-cells technique,
and place a stringent upper limit of bg < 3.5 that is not similar to bg values
of massive DRGs and SMGs but comparable to those of UV-bright sBzKs and LBGs.
Moreover, in optical and near-infrared (NIR) deep fields, we identify
optical-NIR counterparts for 59% of our faint ALMA sources, majority of which
have luminosities, colors, and the IRX-beta relation same as sBzKs and LBGs. We
thus conclude that about a half of our faint ALMA sources are dust-poor high-z
galaxies as known as sBzKs and LBGs in optical studies, and that these faint
ALMA sources are not miniature (U)LIRGs simply scaled down with the infrared
brightness.
