  An improved method is utilized to estimate the X-ray power spectral densities
(PSD) and the variation time scales of three quasars in the Lockman Hole field.
Five archival ROSAT PSPC data covering two year range are analyzed. To estimate
PSD from sparse and unevenly-sampled lightcurves, a forward-method approach
with extensive Monte-Carlo simulations is adopted. A broken power-law type PSD
with a constant Poisson noise component is assumed with a break frequency
$f_{\rm b}$. Then, assuming the PSD slope $\alpha$ as $-2<\alpha<-1$, $1/
f_{\rm b}$ is constrained as $\gtrsim$ 25 days for one object, while the
constraints on the other two objects are very weak. The long time scale of the
one object is consistent with the view that luminous AGNs host massive black
holes.
