PURPOSE: To improve the diffraction efficiency of a crystal by impressing an AC bias electric field to light refracting crystal, selecting the intensity ratio of two beams projected on the crystal to a proper value and properly correcting the frequency and intensity of the AC bias electric field.

CONSTITUTION: The bias electric field at a prescribed frequency is impressed to a light refracting crystal medium 10, and two beams 17 and 20 having the intensity ratio within the range from 0.001 to 1 are projected to the medium 10. Then, based on the interference of two light beams 17 and 20, either of light beams 17 or 20 is modulated by information to be stored as a hologram generated inside the medium 10 and on the conditions of prescribed frequency and intensity ratio, the value of the bias voltage is maintained. Thus, the sensitivity of crystal can be made maximum by maximizing the diffraction efficiency of light refracting crystal, and the hologram can be satisfactorily generated and held.