A servo apparatus for which the servo operation remains stable even if the power supply voltages changes, and the control processing load and amount of control data necessary is reduced by setting up gain variation equipment so as to proceed phase compensators, or setting up gain variation equipment so as to proceed selectors, so that the gain set-up at the time of non-loop operations such as focus searches and track jumps etc. can be varied by the gain variation equipment and the extent of variation of the gain variation equipment can be controlled simultaneously for a number of servo systems.