A circuit is disclosed which is applied for a ditial audio disk (DAD) system for detecting a maximum inverting period of a digital audio signal (102) optically read out from the DAD. The audio signal is prestored in the DAD so as to have the maximum and minimum inverting periods specially set by an eighteen to fourteen modulation (EFM). The detection circuit includes an edge detector (104) for detecting pulse edges of the digital audio signal (102), a counter (106) for counting pulse edge intervals on the basis of a modulating clock signal (108), a counter type register (114), and a comparator (112). When the register contents of the counter type register (114) is smaller than the count value of the counter (106), the comparator (112) produces a pulse signal (118) by which said register (114) updates the contents of the register by "1". Repeating this operation, a maximum inverting period value of the digital audio signal (102) is obtained in a fixed period of time.