A maximum mark length detector comprising a total value register for storing a total value Lsum of measured mark lengths; a measured value register for storing a measured value of a mark length; an arithmetic unit for computing the sum Lp of a current measured value Lk and a previously measured value PLk stored in the measured value register and subtracting a maximum value Lmax from the total value Lsum when the total value Lsum reaches the measured maximum value Lmax; a counter to count the number of times the subtraction is performed; and a comparator for completing the detection when a count value C of the counter reaches a predetermined value.