  The long-term optical and near infrared monitoring observations for a type 1
act ive galactic nucleus NGC 4151 were carried out for six years from 2001 to
2006 b y using the MAGNUM telescope, and delayed response of flux variations in
the $K(2.2\mu m)$ band to those in the $V(0.55\mu m)$ band was clearly
detected. Based on cross correlation analysis, we precisely measured a lag time
$\Delta t$ for eight separate periods, and we found that $\Delta t$ is not
constant changing be tween 30 and 70 days during the monitoring period. Since
$\Delta t$ is the ligh t travel time from the central energy source out to the
surrounding dust torus, this is the first convincing evidence that the inner
radius of dust torus did ch ange in an individual AGN. In order to relate such
a change of $\Delta t$ with a change of AGN luminosity $L$, we presented a
method of taking an average of th e observed $V$-band fluxes that corresponds
to the measured value of $\Delta t$, and we found that the time-changing track
of NGC 4151 in the $\Delta t$ versus $L$ diagram during the monitoring period
deviates from the relation of $\Delta t \propto L^{0.5}$ expected from dust
reverberation. This result, combined with t he elapsed time from period to
period for which $\Delta t$ was measured, indicat es that the timescale of dust
formation is about one year, which should be taken into account as a new
constraint in future studies of dust evolution in AGNs.
