Rewritable optical phase-change discs, such as CD-RW and DVD-RW are now available to store movies, musics and business files. However, the high-speed switching mechanism due to the phase transition has not well been understood. In CAN-FOR, we recently cleared the mechanism by use of a synchrotron radiation beam line in Spring-8. The phase transition in writing and erasing happens due to a small shift of Ge atom in between an off-central position and a tetrahedral bonding position surrounded by neighbor Te atoms.