A disk recording/reproducing apparatus includes a disk rotator which rotates an optical disk, an optical pickup, a slide motor which moves the optical pickup into a radial direction of the optical disk, and a controller which controls at least the disk rotator and the slide motor in accordance with a signal obtained from the optical pickup. The controller determines an alternation destination for recording data at an alternation origin therein or during the alternation process depending on a position of the alternation origin on the optical disk and a reference position on the optical disk, when the optical disk has an alternation area on an innermost periphery and an outermost periphery, and the recording/reproducing apparatus conducts the alternation process upon detection of a defect block within a user data recording area on the optical disk, thereby conducting the alternation process thereon.