  Full radiative transfer in 3D space has been solved to pursue the
reionization history in an inhomogeneous universe. It has been shown that the
reionization of an inhomogeneous universe is not a prompt event, but a fairly
slow process. Also, QSO absorption line systems are simulated with using the
results of radiative transfer calculations. Ly$\alpha$ continuum depression
implies that the metagalactic UV intensity decreases rapidly with $z$ at $z>5$,
and the reionization must have taken place between $z=6$ and 10. Finally, it is
stressed that H$\alpha$ forest is a more powerful tool to probe the
reionization history and the density fluctuations in the universe at $z>5$.
