A method and an optical disk apparatus that can accurately and stably correct the spherical aberration even for a blank write-once optical disk and can detect a spherical aberration error signal with a simple construction of parts in real time. In a blank write-once disk 20, the tracking servo is switched to an inoperative state, push-pull signals from outputs of the photodetector 12 are detected in a state of "in focus" and in a state of "out focus," respectively, by a push-pull signal generation circuit 34, and.the amplitudes of these push-pull signals are subjected to difference calculation in a spherical aberration error detector 31 to obtain a spherical aberration error signal A. Then, the spherical aberration is corrected with this spherical aberration error signal A, and subsequently a specific pattern signal is recorded in a trial writing area, this signal is reproduced in the state of "in focus" and in the state of "out focus," respectively, the amplitude of each reproduced signal is acquired by a reproduced signal generation circuit 32, these amplitudes are subjected to difference calculation in the spherical aberration error detector 31 to obtain a spherical aberration error signal B. The spherical aberration is corrected with this.