PURPOSE: To reproduce signal without any decoding deterioration of Viterbi signal by executing the Viterbi signal while compansating the amplitude fluctuation content even if the amplitude of a read signal fluctuates due to a mechanical position error on reproduction from a recording medium or a characteristic fluctuation of the recording medium.

CONSTITUTION: An optical pickup 1 detects the amplitude value of a read signal (p) based on each obtained sample value when tracing a mirror-surface part and a clock pit provided within a servo area of an optical disk 3 and divides each sample value of a sample value series (q) based on the detected amplitude value, thus obtaining a row of amplitude compensation sample values k. When the detected sample value is large, each sample value of series (q) is divided by the large amplitude value. When the detected amplitude value is small. the sample value of the series (q) is divided by the small amplitude, thus obtaining the series (k) where the amplitude fluctuation is compensated. Therefore, even if the signal (q) fluctuates, the amplitude of the series (k) becomes constant and a Viterbi decoder 20 reproduces digital signals without deteriorating decoding performance.