PURPOSE: To speed up the crystallization and phase transition of a crystal layer by using an alloy expressed by the specific formula to form a recording layer which is reversibly changed in phase between different crystal layers when the layer is irradiated with a light beam.

CONSTITUTION: The recording layer is formed of the alloy having the compsn. expressed by the formula In50-xSb50-xAu2x, where (x) is defined within a 0<x≤5 range. The phase transition is generated between the phase in which the fine crystal of an InSb intermetallic compd. and the fine crystal of Au are mixed and the phase in which the fine crystal of the InSb intermetallic compd. and the coarse crystal grains of Au are mixed by the irradiation conditions of the beam when said layer is projected with the light beam. Namely, the phase in which the fine crystal of the InSb intermetallic compd. and the fine crystal of Au are mixed is formed at the time of initialization and erasing and the phase in which the coarse crystals of Au are mixed is formed at the time of information recording. The reading of information is, therefore, possible if the difference in the reflectivity is detected between these two phase transitions.