{TAIL ~ 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 an optical disk; and 
{ TAIL ~ 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 an optical disk ; and
TAIL TAIL 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 HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH