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We utilize mid-infrared multi-epoch data from Wide-field Infrared SurveyExplorer (WISE) over a $\sim10$-year period in W1 (3.4$\mu$m) and W2(4.6$\mu$m) bands to investigate the structure of dusty torus in low-redshift($0.15<z\le 0.4$) active galactic nuclei (AGN). We calculate a Spearmancorrelation coefficient ($r_{12}$) between W1 magnitude and W1$-$W2 color basedon the light curve in individual objects. Interestingly, $r_{12}$ spans a broadrange from $-1$ to $1$, and is detected to be correlated with mean W1$-$W2color and AGN bolometric luminosity, in the sense that objects with blueW1$-$W2 color and low AGN luminosity tend to become redder (bluer) withincreasing (decreasing) W1 brightness in the light curve (i.e. $r_{12} < 0$),although the correlation of $r_{12}$ with the bolometric luminosity isrelatively weak. The fit for the spectral energy distribution reveals asignificant contribution from the host galaxy in the W1 and W2 bands. However,the dependencies of $r_{12}$ on W1$-$W2 color and AGN luminosity still persisteven after careful elimination of the host light contribution. We propose thatthis result can be explained if covering factor of the hot dust componentdecreases as AGN luminosity increases.

Mid-infrared Variability of Low-redshift Active Galactic Nuclei : Constraints on a Hot Dust Component with a Variable Covering Factor