An optimal recording exposure is determined by varying in the first to nth stages the recording exposure with a write laser beam to record a bright pattern image and a dark pattern image in each stage as the first to nth bright pattern images and the first to nth dark pattern images, respectively; irradiating the respective pattern images with a read beam to detect the intensity of a diffracted beam from the central portion of each image of the bright and dark patterns; calculating Sa1/Sb1=SNR1, . . . , and San/Sbn=SNRn, where Sa1 to San are the intensity of a diffracted beam from the first to nth bright pattern images and Sb1 to Sbn, are the intensity of s diffracted beam from the first to nth dark pattern images; and determining a recording exposure which is given at the SNRmax being the maximum value of the resulting SNR1 to SNRn.