PURPOSE: To easily execute the output as a multivalued code by detecting the signal detected from optical disk by a reference intensity signal of plural levels which can measure amplitude of a vibration part at the rise point of clock and encoding a level of a detecting signal as signal intensity of a reproducing signal.

CONSTITUTION: A signal detected from an optical disk 10 by an optical pickup 12 is detected by reference intensity signals (a), (b) and (c) of plural levels which can measure amplitude of a vibration part by a demodulation circuit 16 at the rise point of a clock and a level of a detecting signal is encoded as signal intensity of a reproducing signal. In such a way, each mark itself can be reproduced easily as a multivalued expression from, therefore, comparing with a conventional binary one, density of information can be enhanced, and also, a processing speed of the signal can be increased.