a substrate;

a first recording layer disposed on the substrate is in the as-deposited states with areas of recording marks formed by as-deposited recording and crystalline areas which have undergone erasure and configured to undergo a reversible phase change between written amorphous and crystalline states due to light irradiation and thereby change an optical characteristic,

wherein said first recording layer includes in the amorphous state and in an as-deposited state an amorphous medium without crystalline order and has dispersed therein a plurality of fine nuclei having an average size of 0.5 nm to 4 nm, and

wherein said first recording layer in an irradiation area exposed to an erasing light beam is in the crystalline state and has a crystal part including a plurality of first crystal grains surrounded by second crystal grains that result from growing the plurality of fine nuclei and having a size of 20 to 100 nm, the first crystal grains being relatively larger than the second crystal grains, the second crystal grains having a size of 4 to 20 nm, and the first crystal grains being evenly distributed throughout said erased areas of said first recording layer among the second crystal grains.