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The long-term optical and near infrared monitoring observations for a type 1act ive galactic nucleus NGC 4151 were carried out for six years from 2001 to2006 b y using the MAGNUM telescope, and delayed response of flux variations inthe $K(2.2\mu m)$ band to those in the $V(0.55\mu m)$ band was clearlydetected. Based on cross correlation analysis, we precisely measured a lag time$\Delta t$ for eight separate periods, and we found that $\Delta t$ is notconstant 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 thesurrounding dust torus, this is the first convincing evidence that the innerradius of dust torus did ch ange in an individual AGN. In order to relate sucha change of $\Delta t$ with a change of AGN luminosity $L$, we presented amethod of taking an average of th e observed $V$-band fluxes that correspondsto the measured value of $\Delta t$, and we found that the time-changing trackof NGC 4151 in the $\Delta t$ versus $L$ diagram during the monitoring perioddeviates from the relation of $\Delta t \propto L^{0.5}$ expected from dustreverberation. This result, combined with t he elapsed time from period toperiod for which $\Delta t$ was measured, indicat es that the timescale of dustformation is about one year, which should be taken into account as a newconstraint in future studies of dust evolution in AGNs.

Variation of Inner Radius of Dust Torus in NGC4151