an irradiation beam photo detector for producing an amount of irradiation beam detection signal according to the amount of a laser beam irradiated from said {HEAD ~ laser beam source} onto {TAIL ~ an optical disk}; and 
onto
an irradiation beam photo detector for producing an amount of irradiation beam detection signal according to the amount of a laser beam irradiated from said { HEAD ~ laser beam source } onto { TAIL ~ an optical disk } ; and
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 HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH