Abstract: In a servo system for a hard disk drive, a malfunction detection circuit monitors a period of a sync signal contained in servo data and outputs a malfunction detection signal indicating that there is a malfunction relating on the servo data. The malfunction detection circuit delivers a malfunction detecting signal when a generation period of a sync signal is longer or shorter than a predetermined cycle initially set. The malfunction detecting circuit includes a pulse generator for generating a pulse signal which is synchronized with the sync signal and first and second pulse generators for measuring a period of the sync signal.