An information recording and reproducing apparatus records and reproduces information on and from an optical recording disk having a sector recording format of frame structure in which data in each sector are divided into a plurality of frames, and a data head identification mark is affixed to the head of each of the frames, so as to prevent propagation of a demodulation error due to failure of word synchronization attributable to pull-out of a PLL during data reproduction. In the apparatus, one frame pulse is generated for each of the frames on the basis of a sector identifier detection signal and a frame data demodulation start signal, and, according to the result of counting the frame pulses, the frame numbers of frame data demodulated and stored in a sector buffer memory are accurately detected. The frame pulse is generated on the basis of the sector identifier detection signal until the data head identification signal is first detected, and, once the data head identification signal is detected, the frame pulse is generated on the basis of the data demodulation start signal. Therefore, even if the data head identification signal of a frame cannot be detected resulting in dropout of data in that frame, the succeeding frame data can be stored at the assigned addresses in the sector buffer memory.