A phase change optical disk using a new quaternary system of Ag-In-Sb-Te as the active layer will be reported. With this disk, "complete erasure" of the recorded signal has been achieved. A carrier-to-noise ratio of 47 dB and an erase ratio of -47 dB were obtained in the two-pass writing-erasing mode with a linear velocity of 7 m/s and a writing power of 11 mW. In the one-pass overwriting mode, a carrier-to-noise ratio greater than 40 dB was obtained and this recorded signal was also completely erased. The peak power and the bias power for the overwriting were 10 mW and 5 mW, respectively. The facts that the widths of the written marks are narrow and that no large crystalline grains are observed in the periphery of the written marks are considered to be the major causes for the high erasability. We also found that the existence of fine crystalline particles in the initialized active layer can be considered to be of great advantage to high sensitivity.