To shorten data transfer time by inserting a purple color film or org. photochromic material into the optical disk of a resonator structure and reading out the contents of a memory by the wavelength of selectively transmitted light at the time of simultaneous irradiation with two wavelengths.

The change rate of the Fabry-Perot resonator may be changed by a change in the complex index of refraction of the photochromic material 19 if this resonator is constituted by holding the photochromic material 19 between two sheets of translucent mirrors 20. If the photochromic material 19 is irradiated with the light 21 of the wavelength λ1 when this material is in the state A, the greater part of the light is reflected like arrows 22. If, however, the irradiation is sustained, the photochromic material 19 is converted to a state B by a phtoochemical reaction and the complex index of refraction is changed and, therefore, the incident light 21 of the wavelength λ1 passes the resonator. As a result, data '1' is recorded in the photochromic material 19.