An optical disc is formed by bonding a first information recording medium to a second information recording medium. The first information recording medium includes a first substrate having opposed first and second surfaces, at least one dielectric film, at least one recording film, and a cover layer. The second information recording medium includes a second substrate having opposed third and fourth surfaces, at least one dielectric film, at least one recording film, and a cover layer. The second surface of the first substrate is bonded to the fourth surface of the second substrate. At least one of the second surface or the fourth surface includes a depression and a projection. The depression is formed in a thickness direction of the optical disc. The projection is provided inside the depression at a predetermined distance or longer from an outer circumferential edge of the depression in a radial direction of the optical disc.

A recording-medium binding device includes first and second tooth groups. The first tooth group includes first and second tooth rows. The second tooth group includes third and fourth tooth rows. The third tooth row to be engaged with the first tooth row forms, together with the first tooth row, a first tooth row pair. The fourth tooth row to be engaged with the second tooth row forms, together with the second tooth row, a second tooth row pair. A second engaging height in the second tooth row pair is smaller than a first engaging height in the first tooth row pair. A second effective tooth top width in the second tooth row pair is larger than a first effective tooth top width in the first tooth row pair.

A recording-medium binding device includes first and second tooth groups. The first tooth group includes first and second tooth rows. The second tooth group includes third and fourth tooth rows. The third tooth row to be engaged with the first tooth row forms, together with the first tooth row, a first tooth row pair. The fourth tooth row to be engaged with the second tooth row forms, together with the second tooth row, a second tooth row pair. A second engaging height in the second tooth row pair is smaller than a first engaging height in the first tooth row pair. A second effective tooth top width in the second tooth row pair is larger than a first effective tooth top width in the first tooth row pair.

An energy curable bonding resin composition that prevents corrosion in metallic optical structures including DVDs includes at least one monomer, acrylated epoxidized soya bean oil, and a photoinitiator of among 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO); Phosphine oxide, phenylbis 2,4,6-trimethyl benzoyl; oligo[2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propanone]; alpha-hydroxy ketone, difunctional; or combinations thereof.

An energy curable bonding resin composition that prevents corrosion in metallic optical structures including DVDs includes at least one monomer, acrylated epoxidized soya bean oil, and a photoinitiator of among 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO); Phosphine oxide, phenylbis 2,4,6-trimethyl benzoyl; oligo[2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propanone]; alpha-hydroxy ketone, difunctional; or combinations thereof.

Provided is an optical recording medium including: two discs, each of which includes a substrate and a plurality of recording layers; and an attachment layer which is provided between the two discs and includes ultraviolet curable resin, wherein thickness of each of the substrates in the two discs is equal to or greater than 0.3 mm and equal to or less than 0.545 mm, wherein transmittance of the two discs is equal to or less than 20%, and wherein inversion rates on both surface sides of the attachment layer are equal or substantially equal to each other, and are equal to or greater than 70%.

Provided is an optical recording medium including: two discs, each of which includes a substrate and a plurality of recording layers; and an attachment layer which is provided between the two discs and includes ultraviolet curable resin, wherein thickness of each of the substrates in the two discs is equal to or greater than 0.3 mm and equal to or less than 0.545 mm, wherein transmittance of the two discs is equal to or less than 20%, and wherein inversion rates on both surface sides of the attachment layer are equal or substantially equal to each other, and are equal to or greater than 70%.