PURPOSE: To read each information signal on a recording surface which is laminated plural times by irradiating the laser light of a single wavelength from a single laser light source and accordingly providing a means to produce the laser light of plural wavelengths.

CONSTITUTION: Two beams 11A an 11B are obtained by using a laser light source 10B together with an optical crystal 17 and a demultiplexer 18 set between the source 10B and lens systems 12A and 12B. The crystal 17 is used for conversion of frequency. When the light of wavelength λ is irradiated to the crystal 17, the light of wavelength λ and the light of λ1 wavelength are produced. Thus the crystal 17 produces a beam 11B and a beam 11A of 1/2 wavelength of the beam 11B when the beam 11B is irradiated to the crystal 17. Then these two beams 11B and 11A are made incident to the demultiplexe 18. Thus it is possible to read each information signal out of recording surfaces 2A and 2B of a disk 1.