To provide a method and apparatus for measuring an ionization rate, which is capable of easily measuring the ionization rate of plasma and thus is capable of achieving improvement in productivity and efficiency of surface treatment using plasma.

A voltage making a potential equal to or lower than substantially a plasma potential is applied to a base material 10 for measurement, and a current flowing through the base material 10 for measurement is measured, thereby calculating the ionization rate of plasma 5 using the film thickness of a film deposited on the base material 10 for measurement and the measurement result of the current.