  Recent studies of active galactic nuclei (AGN) found a statistical inverse
linear scaling between the X-ray normalized excess variance $\sigma_{\rm
rms}^2$ (variability amplitude) and the black hole mass spanning over $M_{\rm
BH}=10^6- 10^9\ M_{\odot}$. Being suggested to have a small scatter, this
scaling relation may provide a novel method to estimate the black hole mass of
AGN. However, a question arises as to whether this relation can be extended to
the low-mass regime below $\sim10^6\ M_{\odot}$. If confirmed, it would provide
an efficient tool to search for AGN with low-mass black holes using X-ray
variability. This paper presents a study of the X-ray excess variances for a
sample of AGN with black hole masses in the range of $10^5- 10^6\ M_{\odot}$
observed with {\it XMM-Newton} and {\it ROSAT}, including data both from the
archives and from newly preformed observations. It is found that the relation
is no longer a simple extrapolation of the linear scaling; instead, the
relation starts to flatten at $\sim10^6\ M_{\odot}$ toward lower masses. Our
result is consistent with the recent finding of \citet{L15}. Such a flattening
of the $M_{\rm BH}-\sigma_{\rm rms}^2$ relation is actually expected from the
shape of the power spectrum density of AGN, whose break frequency is inversely
scaled with the mass of black holes.
