An optical memory device for recording, reproducing, or erasing information applies an optical beam such as a laser beam onto a recording medium. The optical memory device includes a recorded region detecting circuit for detecting information recorded region formed in the recording medium. In the optical memory device, a timing control for recording, reproducing, or erasing is performed with the use of a result obtained through detection of a synchronizing signal, such as a sector mark, indicative of a starting point of an information block and another result obtained through detection in the recorded region detecting circuit. The recorded region detecting circuit includes either at least one peak hold circuit for detecting a peak level of the playback signal or a low pass filter for smoothing the playback signal, and a comparator for comparing an output obtained through the peak hold circuit or through the low pass filter with a reference voltage.