OPTICAL DEVICE

PROBLEM TO BE SOLVED: To prevent a focus search operation position from being limited to an inner periphery or a multilayer recording medium speed from decreasing by performing a macro or micro focus follow-up operation based on a focus error signal. SOLUTION: Macro and micro focus error signals MAF and MIF by obtaining the difference between the output of macro/micro focus photodetectors 9 and 8 by subtractors 10 and 11 enter gain control circuits 12 and 14, and optimize a micro focus loop gain according to the instruction of a control circuit 23. Optimized focus error signals MAFE and MIFE are switched by a focus loop switching circuit 17 according to the instruction of the control circuit 23, are subjected to phase compensation by a phase compensation circuit 18, and then are reduced to a lower band than the rotary frequency of a recording medium 29 by a low-region down circuit 19 according to the instruction of the control circuit 23, and are inputted to a loop on switch 20, thus performing micro focus follow-up control and at the same time initializing the disk 29 by moving the multilayer disk 39 with a head 30.