in device for switching the focus control device from an inactive state to an active state a device for moving an irradiation position of the light beam in a surface of the medium to effect tracking control and a device for holding the light beam at a center of a movable range by the tracking control device during the focus pull in application_number US 1322793 A description fig1 shows an embodiment of the present invention applied to an optical disk apparatus an optical disk 31 is rotated by a spindle motor 32 an optical head 33 for recording and reproducing information on and from the optical disk 31 is moved radially of the disk by drive means not shown a semiconductor laser 34 is provided in the optical head 33 a light beam emitted from the semiconductor laser 34 passes through a collimater lens 35 and a beam splitter 36 and is focused onto the optical disk 31 by an objective lens 1 the light reflected by the optical disk 31 passes through the objective lens 1 and is reflected by a beam splitter 36 the reflected beam is focused by a sensor lens 38 which comprises a rotating symmetrical lens and a cylindrical lens and is detected by a four element detector 8 which detects a focusing error signal.