In an information recording/reproducing apparatus, frame synchronization is speedily performed when initialization is executed or the recording surfaces are switched, and the reliability of recorded data is improved. Segment IDs, each being formed by coding an information word representing a rotational angle measured from a rotational angle origin, are provided within the respective servo areas in such a manner that they are spaced equally in the track direction. While initial synchronization is performed, frame synchronization is established based on the information obtained by decoding the segment ID. This eliminates the need for detecting a home index pattern, thereby speedily performing the initial synchronizing operation. After initial synchronization is achieved, it is constantly monitored whether a frame is not out of synchronization based on the information obtained by decoding the segment ID. When it is detected that a frame is out of synchronization, a suitable operation is performed to minimize the destruction of recorded data.