Abstract: A focus error amplitude obtaining device is configured to obtain amplitudes of a focus error signal. An error amplitude difference calculation device is configured to calculate an error amplitude difference which is a difference between the amplitudes of the focus error signal obtained at two different positions in a moving range of a spherical aberration correction device. An appropriate position search control device is in charge of overall control of structural elements; spherical aberration correction device movement control device, focus error amplitude obtaining device, and error amplitude difference calculation device. The appropriate position search control device decides an appropriate position of the spherical aberration correction device based on the error amplitude difference.