The present invention relates to a magnetic disk substrate formed of a thermoplastic resin and a magnetic disk comprising a magnetic recording layer formed on the magnetic disk substrate. In the magnetic disk substrate of the invention, the amount of gases generated therefrom upon held at 90 C. for 1 hour is 100 g/cm² or less, so that when scanned by a floating head, errors on recording and reading of information are remarkably reduced. Thus, the magnetic disk substrate can provide a magnetic disk having improved scan capability. The present invention also provides a process for manufacturing such a magnetic disk.