Patent ID: 7263057

Claim:
An optical disc for recording information between grooves of the disc, comprising: a resin substrate having the grooves on a surface of the substrate; a reflection layer formed over the surface of the resin substrate such that the reflection layer has a concavo-convex shape corresponding to the grooves; an optical-characteristic-changeable layer formed over the reflection layer such that the optical-characteristic-changeable layer has a concavo-convex shape corresponding to the concavo-convex shape of the reflection layer, the optical characteristic-changeable layer having an optical characteristic which varies when a light beam is radiated on the optical-characteristic-changeable layer; a light-transmissive protection layer formed over the optical characteristic-changeable layer; and a light-transmissive cover layer formed over the protection layer for sealing the reflection layer, optical characteristic-changeable layer and protection layer to the resin substrate, wherein the light beam creates a beam spot on the convex portion of the optical-characteristic-changeable layer, such that the following equation is satisfied: ( Sb×Lb×Cb )≧( Sa×La×Ca ), where: Sb represents a square of a diameter of the beam spot, Lb represents a thickness of the protection layer, Cb represents a thermal conductivity of the protection layer, Sa represents the square of the beam spot diameter, La represents a thickness of the reflection layer, and Ca represents a thermal conductivity of the reflection layer, such that heat, which is generated at a convex portion of the optical characteristic-changeable layer by the light beam radiated to the convex portion, is transferred from the convex portion toward the protection layer in a greater amount than from the convex portion toward the reflection layer.