In a focus optical system for optical discs or optical disc master exposure apparatuses, a relative positional relation between a cylindrical lens and a 4-division detector in an astigmatic optical system is adjust so that an interference fringe appearing in reflected light derived from an optical disc master is made to be incident on a dead zone of the 4-division detector. Thus, an astigmatic focus servo free from effects of variations in a focus error signal due to the interference fringe is realized. As a result of this, effects of the interference fringe appearing in the reflected light derived from an optical disc or optical disc master can be suppressed.