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 $K(2.2mu m)$ band to those in the $V(0.55mu m)$ 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.
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