Additionally, the ?? value is a parameter relating to a reproduced signal level or amplitude, and is obtained by (a+b)/(a???b) in which character a denotes a peak level (with a sign of +) of an eight to fourteen modulation (EFM) signal waveform as a returned light receiving signal of an optical pickup, and b (with a sign of ???) denotes a bottom level of the returned signal. [0011] Additionally, i