  It is not yet clear what triggers the activity of active galactic nuclei
(AGNs), but galaxy merging has been suspected to be one of the main mechanisms
fuelling the activity. Using deep optical images taken at various ground-based
telescopes, we investigate the fraction of galaxy mergers in 39 luminous AGNs
(M$_{R}\, \lesssim$ -22.6 mag) at $z \leq$ 0.3 (a median redshift of 0.155), of
which the host galaxies are generally considered as early-type galaxies.
Through visual inspection of the images, we find that 17 of 39 AGN host
galaxies (43.6%) show the evidence for current or past mergers like tidal
tails, shells, and disturbed morphology. In order to see if this fraction is
abnormally high, we also examined the merging fraction of normal early-type
galaxies in the Sloan Digital Sky Survey (SDSS) Strip 82 data (a median
redshift of 0.04), of which the surface-brightness limit is comparable to our
imaging data. To correct for the effects related to the redshift difference of
the two samples, we performed an image simulation by putting a bright point
source as an artificial AGN in the images of SDSS early-type galaxies and
placing them onto the redshifts of AGNs. The merging fraction in this realistic
sample of simulated AGNs is only $\sim 5 - 15\%$ ($1/4$ to $1/8$ of that of
real AGNs). Our result strongly suggests that luminous AGN activity is
associated with galaxy merging.
