A method of detecting a characteristic (such as thickness) of first and second films formed on a substrate includes supplying a sensor coil with a first alternating current having a first frequency when detecting the first film, and a second alternating current having a second frequency when detecting the second film. An eddy current is thereby generated in the first film or the second film. An impedance across the sensor coil is measured, and the characteristic of any one of the first film and the second film is detected based on the impedance.