To reduce the whole measuring time with a time-division rate by performing the measurement in the manner of periodically changing the measuring condition to the time-division within the specified period.

The detection is executed while changing focus bias voltages (A, B, C) with the time-division manner, e.g. for every 2 ms taking the rotational period of an optical disk as e.g. 120 ms. Next, when the measurements for one rotation of the optical disk are finished, the jitter amounts of an RF signal are measured while changing three focus bias voltages (D, E, F) with the time- division manner this time. Further, by successively changing over respective three focus bias voltages (G, H, I) (J, K, L) (M, N, O) for every one rotation of the optical disk, the jitter amounts of the RF signal are measured with each time-division. By this procedure, the jitter amounts of the RF signals for every 20 points during one rotation of the optical disk are measured e.g. for each of three focus bias voltages.