  We present the measurement of the line-of-sight extinction of the dusty torus
for a large number of obscured active galactic nuclei (AGNs) based on the
reddening of the colour of the variable flux component in near-infrared (NIR)
wavelengths. We collected long-term monitoring data by $\textit{Wide-field
Infrared Survey Explorer (WISE)}$ for 513 local AGNs catalogued by the
$\mathit{Swift/}$BAT AGN Spectroscopic Survey (BASS) and found that the
multi-epoch NIR flux data in two different bands (WISE $W1$ and $W2$) are
tightly correlated for more than 90% of the targets. The flux variation
gradient (FVG) in the $W1$ and $W2$ bands was derived by applying linear
regression analysis, and we reported that those for unobscured AGNs fall in a
relatively narrow range, whereas those for obscured AGNs are distributed in a
redder and broader range. The AGN's line-of-sight dust extinction ($A_V$) is
calculated using the amount of the reddening in the FVG and is compared with
the neutral hydrogen column density ($N_{\rm{}H}$) of the BASS catalogue. We
found that the $N_{\rm{}H}/A_V$ ratios of obscured AGNs are greater than those
of the Galactic diffuse interstellar medium (ISM) and are distributed with a
large scatter by at most two orders of magnitude. Furthermore, we found that
the lower envelope of the $N_{\rm{}H}/A_V$ of obscured AGNs is comparable to
the Galactic diffuse ISM. These properties of the $N_{\rm{}H}/A_V$ can be
explained by increase in the $N_{\rm{}H}$ attributed to the dust-free gas
clouds covering the line of sight in the broad-line region.
