An optical information recording/reproducing apparatus for recording an interference pattern between signal light and reference light information-multiplexed as a hologram in an optical information recording medium and for reproducing information recorded in the optical information recording medium on the basis of the recorded hologram with use of the reference light. The apparatus includes a light source for emitting light and a coherency controller for controlling a coherency of the light source. The apparatus controls the coherency controller in such a manner as to switch between a coherency required in a recording mode and a coherency required in a reproducing mode.