Long-term X-ray variations of black hole candidates from MAXI observations

MAXI discovered not only more than ten black hole candidates, but also recurrent black hole X-ray novae. MAXI monitored longterm variations and state transitions of these black hole binaries. Now, we have enough samples to comfirm previous findings, e.g., histerisis behavior in state transitions (Miyamoto et al. 1995) or Q-curves (Fender et al. 2004), and a definite Eddington rate of the luminosity at the soft-to-hard state transitions (Maccarone 2003). We also show recurrent intervals of the X-ray novae. Together with the number of known black hole binaries, we try to estimate a total number of black holes in our galaxy.