Based on an output signal of a sector mark reading circuit reading a sector mark signal representing a sector mark area, the interval in which the next sector mark signal is to be detected is anticipated, and a quasi-sector mark is generated at a time when the next sector mark is to be detected in each sector. Even if the sector mark reading fails, the sector mark will be able to be output at an accurate timing. Also, if the read sector mark signal, which is mis-detected or not, is determined, a highly reliable sector mark signal may be produced.