A phase-change optical storage medium has a substrate, and a recording layer, to be recorded on which is at least one recorded mark representing information to be recorded by irradiating a recording light beam onto the recording layer in accordance with a recording pulse pattern of recording pulses rising from an erasing power and formed between a recording power larger than the erasing power and a bottom power smaller than the erasing power and of erasing pulses rising from the bottom power to the erasing power. The expressions (1) and (2): 1.00<(R1/R0)<1.15 . . . (1), 1.05<(R9/R0)<1.20 . . . (2) are satisfied for the recording layer, in which R0 is a reflectivity exhibited by an un-recorded section of the recording layer, on which no data has ever been recorded, when irradiated with a reproducing light beam, R1 is a reflectivity exhibited by the un-recorded section when irradiated with the reproducing light beam, after irradiated once with the recording light beam in accordance with the recording pulse pattern, and R9 is a reflectivity exhibited by the un-recorded section when irradiated with the reproducing light beam, after irradiated nine times with the recording light beam in accordance with the recording pulse pattern.