PURPOSE: To surely earth a conductive holding member or the like around an encoder unit even at the time when the encoder unit is moved toward or away from a lens unit for the purpose of focusing by an encoder and to surely fix a substrate.

CONSTITUTION: A slide projection 88 is slide in a slide groove 86, and an encoder unit EU is moved toward or away from a lens unit LU, and the optical path length of a synchronizing laser beam is adjusted to focus on a linear encoder plate 41. In this case, a fixing bolt 198 is loosened and is moved in a long hole 184 of a substrate 174 and is fastened in a proper approaching/ separating direction movement position to fix the substrate 174 on the side of the lens unit LU. The fixing bolt 198 is fastened to make a ground pattern 200 electrically conductive to a conductive stand 64 on the side of the lens unit LU, conductive left and right upper plates 120 and 122 on the side of the encoder unit EU, etc.