in height from the bottom surface of the sil in this embodiment an effective portion of the sil bottom surface has a size of 40 u03bcm u00d7 20 u03bcm track direction u00d7 radial direction this is because an ordinary plastic made optical disk substrate tilts mainly in the radial direction due to a change in temperature humidity or the like with the above construction however even when a distance between the sil bottom surface and an optical disk 14 is reduced to around 100 nm the disk and the sil bottom surface will not contact each other in relative inclination up to around 0 56 u00b0 also even when the distance between the sil bottom surface and the optical disk 14 is reduced to around 50 nm an allowable relative inclination is increased to around 0 28 u00b0 in this embodiment the aforementioned step difference in height provided for the sil bottom surface is 1 u03bcm it is sufficient that the step difference is provided with a height which can prevent an edge portion of the sil bottom surface from contacting with the optical disk so there is no need to provide a step difference in height larger than is necessary for instance even when the distance between the sil bottom surface and the optical disk 14 is reduced to around 100 nm and the relative inclination becomes 0 56 u00b0 the sil edge portion maintains a distance of.