  Long-term X-ray variability of the low-luminosity active galactic nucleus of
M81 was studied, using 16 ASCA observations spanning 5.5 years. The object
exhibits a factor three variation over the 5.5 years. The source intensity was
relatively constant within each observation which lasted typically for one day,
but intra-day variability by 30% was detected on the 15th observation. The
power-spectral density (PSD) was estimated in ``forward'' manner, over a
frequency range of 10^(-8.2)--10^(-4.3) Hz (period range of 1/4 day-5.5 years),
by utilizing structure function and extensive Monte-Carlo simulations in order
to overcome the very sparse and uneven data samplings. When the PSD is assumed
to be white below a "break frequency" f_b and falls off as proptional to
f^(-alpha) above f_b, where "f" is frequency and alpha is a positive parameter,
the M81 light curve is well described with 1/f_b > 800 days and alpha = 1.4 +-
0.2.
