  We have studied the evolution of the central hundred pc region of barred
galaxies by performing numerical simulations realizing multi-phase nature of
gas. Our simulations have shown that a stellar bar produces an oval gas ring
namely the $x$-2 ring within $1~{\rm kpc}$ as the bar grows. The ring is
self-gravitationally unstable enough to trigger formations of gas clouds.
Although the gas clouds initially rotate in the $x$-2 ring, cloud-cloud
collisions and/or energy injections into the gas ring by Type II supernovae
deviate some of the clouds from the ring orbit. After the deviation, the
deviated clouds repeat collisions with the other clouds, which rotate in the
$x$-2 ring, during several rotations. These processes effectively reduce the
angular momentum of the deviated gas cloud. As a result, the gas cloud finally
falls into the galactic center, and episodic gas supply to the galactic center
takes place.
