Flexible optical disk

A flexible optical disk, comprising:

• a disk substrate;
• wherein a rigidity of the disk substrate is equal to or more than 5 Pa and equal to or less than 5000 Pa where the rigidity is defined as Young's modulus of the disk substrate×(thickness of the disk substrate/radius of the disk substrate)3,
• wherein the Young's modulus and an internal loss (internal friction) of the disk is defined by a characteristic vibration method that is a dynamic measurement method and a cantilever resonance method,
• wherein the internal loss is defined based on a distribution of a characteristic frequency by a ratio of a frequency and a half value thickness, and
• wherein the internal loss (Q−1=(fU−fL)/(″3×f)) is equal to or more than 0.001 and equal to or less than 0.3 where the characteristic frequency is defined as f, a frequency at a side where the half value thickness is high is defined as fU, and a frequency at a side where the half value thickness is low is defined as fL.