When {HEAD ~ the data} is reproduced by irradiating {TAIL ~ a laser beam} onto the optical disk, an amount of irradiation beam detection signal is produced according to the amount of the laser beam irradiated onto the optical disk, and the defect on the optical disk is detected on the basis of this amount of irradiation beam detection signal 
is reproduced by irradiating
When { HEAD ~ the data } is reproduced by irradiating { TAIL ~ a laser beam } onto the optical disk , an amount of irradiation beam detection signal is produced according to the amount of the laser beam irradiated onto the optical disk , and the defect on the optical disk is detected on the basis of this amount of irradiation beam detection signal
OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH