For recording data on and reproducing data from an optical disk of a single spiral land/groove configuration, a header detector detects header regions on the optical disk, a PID error detector judges whether the address information read from the header regions is erroneous, using error detection codes, and detects the number of errors per sector. An error count comparator compares the number of errors per sector with a predetermined value, and a state judging circuit identifies the state of the optical disk device by causing transition to a higher or lower state according to the output of the error count comparator. The recording and reproduction are controlled according to the state thus identified.