  We aim to find out the reason why there exists an anisotropic HI absorption
around quasars; i.e., the environments around quasars are highly biased toward
producing strong HI absorption in transverse direction while there exists a
significant deficit of HI absorption within a few Mpc of quasars along
line-of-sight. The most plausible explanation for this opposite trend is that
the transverse direction is shadowed from the quasar UV radiation due to dust
torus. However, a critical weakness of this idea is that we have no information
on inclination angle of our sightline relative to the torus. In this study, we
examine environments of quasars with broad absorption troughs in their spectra
(i.e., BAL quasars) because it is widely believed that BAL troughs are observed
if the central continuum is viewed from the side through their powerful
outflows near the dust torus. With closely separated 12 projected quasar pairs
at different redshift with separation angle of $\theta$$<$120$^{\prime\prime}$,
we examine HI absorption at foreground BAL quasars in spectra of background
quasars. We confirm there exist optically thick gas around two of 12 BAL
quasars, and that the mean HI absorption strength is EW$_{\rm rest}$$\sim$1A.
These are consistent to the past results around non-BAL quasars, although not
statistically significant. However, the origins of optically thick HI absorbers
around BAL and non-BAL quasars could be different since their column densities
are different by $\sim$3 orders of magnitude. The larger sample would be
required for narrowing down possible scenarios for the anisotropic HI
absorption around quasars.
