  We propose a mechanism of state transition in wind-fed black hole binaries
(high-mass X-ray binaries) such as Cyg X-1 and LMC X-1. Modeling a line-driven
stellar wind from the companion by two-dimensional hydrodynamical calculations,
we investigate the processes of wind capture by and accretion onto the black
hole. We assume that the wind acceleration is terminated at the He II
ionization front because ions responsible for line-driven acceleration are
ionized within the front, i.e. He III region. It is found that the mass
accretion rate inferred from the luminosity is remarkably smaller than the
capture rate. Considering the difference, we construct a model for the state
transition based on the accretion flow being controlled by magneto-rotational
instability. The outer flow is torus like, and plays an important role to
trigger the transition. The model can explain why state transition does occur
in Cyg X-1, while not in LMC X-1. Cyg X-1 exhibits a relatively low luminosity,
and then the He II ionization front is located and can move between the
companion and black hole, depending on its ionizing photon flux. On the other
hand, LMC X-1 exhibits too high luminosity for the front to move considerably;
the front is too close to the companion atmosphere. The model also predicts
that each state of high-soft or low-hard would last fairly long because the
luminosity depends weakly on the wind velocity. In the context of the model,
the state transition is triggered by a fluctuation of the magnetic field when
its amplitude becomes comparable to the field strength in the torus-like outer
flow.
