A PLL circuit (24) reproduces a clock signal from a reproduction EFM signal of an optical disk (10). A phase error signal (pco) of the PLL circuit (24) is converted into an absolute value by an absolute value circuit (38), and is averaged for each subcode frame period by an accumulator (40). The recording beam power of a laser beam is determined on the basis of an output of the accumulator (40). A pulse-length discrimination circuit (26) discriminates the pulse length of the reproduction EFM signal. A specific-pulse-length phase-error-signal extracting circuit (42) extracts the phase error signal (pco) of a specific pulse length. The extracted phase error signals (pco) are averaged for the respective subcode frame periods by a accumulator (44). The time base of the corresponding pulse length of the recording EFM signal is corrected on the basis of the output of the accumulator (44).