PURPOSE: To improve the reliability of writing data by forming substrates whose rear faces are flatly confronted with recording medium layers, and whose surfaces have parts having thicknesses whose reflectances are different corresponding to reading lights, after connecting their projecting and recessed face sides with the one face of a transparent disk.

CONSTITUTION: Plural recessed parts 6 which are shallower than those to form a preformat pit are prepared on the inside face of the transparent disk, both faces of recording medium layers 3 are formed to be flat, and the one faces of a substrates 2 are flatly connected with the recording medium layers 3, and projecting parts 7 projecting at the recessed parts 6 are formed on the other faces. Therefore, the substrates 2 have different reflectances partially, and preformat information such as synchronous signals can be obtained by the optical signals whose reflectances are different. Also, the recording medium layers 3 connected with the substrates 2 are flat, so that the generation of a crack or the deterioration due to preservation for a long period and temperature and humidity can be prevented. Thus, the reliability of the writing data can be improved.