A magnetooptic storage member is scanned by a single laser beam. The beam intensity is sufficient to heat a localized area of the storage member to be above the Curie temperature for enabling reversing the remanent magnetization of the localized area. Such localized area is subjected to a magnetic bias field for directing which of the remanent directions shall be assumed in the localized area. The storage member and laser beam are relatively moved such that a front or leading portion of the localized area does not reach the Curie temperature. The light reflected from this leading portion, which moves with the laser beam relative to the storage member, is detected for indicating the initial remanent state of the localized area before any changes are made by the combination of the bias field and the laser beam.