A signal processing apparatus for an optical disk system which apparatus has a variable gain amplifier that amplifies with a variable gain a light-detected signal obtained from an optical disk on which to perform playback processing so as to make a level of the light-detected signal coincide with a first reference level. The apparatus comprises a comparator that compares the level of the light-detected signal amplified by the variable gain amplifier with the first reference level; a gain adjuster that generates and supplies a control signal to adjust the variable gain according to a result of the comparing to the variable gain amplifier; and a gain adjustment controller that, when the level of the light-detected signal amplified is within a level range including the first reference level, controls a level of the control signal of the gain adjuster to be held at its preceding value.