An optical disk of the present invention includes a first substrate having a
signal
area on a principal plane and a central hole, and a second substrate that is transparent
and attached to the first substrate. The second substrate is thinner than the first
substrate, and has a central hole whose diameter is larger than that of the first
substrate. The first substrate and the second substrate are attached to each other
with radiation curable resin (adhesive member) disposed therebetween so as to extend
at least from an inner peripheral edge of the second substrate to an outer peripheral
edge thereof.