An optical disk includes a substrate covered with an information recording layer. The information recording layer includes a recording material and a dispersion material dispersed in the recording material. The recording material absorbs recording light for recording information, and then generates heat. As the light refraction index of the recording material changes due to the heat generation, information is recorded on the recording material. The dispersion material has lower absorptance of the recording light and lower thermal conductivity than the recording material, and an external dimension smaller than a diffraction limit of reading light for reading information.