  Monitor of All-sky X-ray Image (MAXI) on the International Space Station has
been observing the X-ray sky since 2009 August 15. It has accumulated the X-ray
data for about four years, so far. X-ray objects are usually variable and their
variability can be studied by the power spectrum density (PSD) of the X-ray
light curves.We applied our method to calculate PSDs of several kinds of
objects observed with MAXI. We obtained significant PSDs from 16 Seyfert
galaxies.For blackhole binary Cygnus X-1 there was a difference in the shape of
PSD between the hard state and the soft state. For high mass X-ray binaries,
Cen X-3, SMC X-1, and LMC X-4, there were several peaks in the PSD
corresponding to the orbital period and the superorbital period.
