To provide a detection method capable of detecting existence of a transparent and hardly discriminated coating layer such as a coating layer formed by coating an adhesive liquid onto the original film during traveling of an original film, and a device.

Thickness of the coating layer of the coating layer formation area of the continuously traveling original film 2 is measured by a laser displacement meter. It is determined whether or not the measured thickness exceeds the minimum thickness required for the coating layer. Thereby, existence of the appropriate coating layer is detected.