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Observations of 226 AGNs in the near-infrared $J$, $H$, and $K'$ bands arepresented along with the analysis of the observations for variability. Oursample consists mainly of Seyfert 1 AGNs and QSOs. About a quarter of theobjects in each category are radio loud. The AGNs in the entire sample have theredshifts 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 wasmeasured by differential photometry.  A reduction method of differential photometry, optimized to the analysis ofextended images, has been developed. The systematic error in variabilityarising from AGNs of highly extended images is estimated to be less than 0.01mag in each of the $J$, $H$, and $K'$ bands. The systematic error arising fromthe flat fielding is negligible for most AGNs, although it is more than 0.1 magfor some particular cases. The overall average flat fielding error is 0.03 magfor the image pairs. We find that these systematic errors are superseded bystatistical errors, and the overall average total systematic and statisticalerrors amounts to 0.05 mag in the measured variability in each band.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 ofmore than $2\sigma$, and 44% of more than $3\sigma$. This result holdsindependent of the $J$, $H$, and $K'$ bands. The detection rate of variabilityis higher for a subsample of higher photometric accuracy, and there appears nolimit to this tendency. In particular, when we consider a subsample with smallphotometric errors of $\sigma<0.03$ mag, the rate of $2\sigma$ detection is80%, and 64% for $3\sigma$ detection. This suggests that most AGNs are variablein the near-infrared

JHK ′ Imaging Photometry of Seyfert 1 Active Galactic Nuclei and Quasars. II. Observation of Long‐Term Variability