  Observations of 226 AGNs in the near-infrared $J$, $H$, and $K'$ bands are
presented along with the analysis of the observations for variability. Our
sample consists mainly of Seyfert 1 AGNs and QSOs. About a quarter of the
objects in each category are radio loud. The AGNs in the entire sample have the
redshifts spanning the range from $z=0$ to 1, and the absolute magnitudes from
$M_B=-29$ to -18. All the objects were observed twice and their variability was
measured by differential photometry.
  A reduction method of differential photometry, optimized to the analysis of
extended images, has been developed. The systematic error in variability
arising from AGNs of highly extended images is estimated to be less than 0.01
mag in each of the $J$, $H$, and $K'$ bands. The systematic error arising from
the flat fielding is negligible for most AGNs, although it is more than 0.1 mag
for some particular cases. The overall average flat fielding error is 0.03 mag
for the image pairs. We find that these systematic errors are superseded by
statistical errors, and the overall average total systematic and statistical
errors amounts to 0.05 mag in the measured variability in each band.
  We find that 58% of all the AGNs in the entire sample show variability of
more than $2\sigma$, and 44% of more than $3\sigma$. This result holds
independent of the $J$, $H$, and $K'$ bands. The detection rate of variability
is higher for a subsample of higher photometric accuracy, and there appears no
limit to this tendency. In particular, when we consider a subsample with small
photometric errors of $\sigma<0.03$ mag, the rate of $2\sigma$ detection is
80%, and 64% for $3\sigma$ detection. This suggests that most AGNs are variable
in the near-infrared.
