Optical storage medium

An optical storage medium includes: a substrate having a first surface and a second surface on both sides, the first surface allowing light to pass therethrough in recording or reproduction; and two or more of composite layers formed on the second surface. Each layer is capable of storing data to be recorded with the light. At least one composite layer, except a composite layer provided farthest from the first substrate, has at least a recording film, a reflective film, a first optical adjustment film and a second optical adjustment film laminated in order over the second surface. Each film allows the light to pass therethrough in recording or reproduction to or from composite layers provided farther from the first substrate than the one composite layer being. The one composite layer satisfies relations: 2.5