The performance of a 0.6-mm-thick substrate disk was studied for tilted disk conditions. It was shown by numerical calculation that the reduction in the peak intensity of the optical spot was 0.5 dB under a disk tilt of 7 mrad when the numerical aperture (NA) of an objective lens was 0.6. The tilt angle dependence of the reading/writing performance was measured for a double-sided disk with a wavelength of 690 nm. Excellent signal quality was obtained, such as carrier-to-noise ratio of over 50 dB, erasability of more than 27 dB, crosstalk of under -27 dB, and jitter of under 30% of the window width for a tilt angle of 7 mrad.