To provide a method for measuring the thickness of a liquid crystalline polymer solution layer applied onto a traveling long substrate film, monitoring the state of application, and controlling an application process.

Light is irradiated from the side of the liquid crystalline polymer solution layer of a long layered product made of a traveling uncured liquid crystalline polymer solution layer/substrate film. On the basis of the optical path difference between reflected light from the surface of the liquid crystalline polymer solution layer and reflected light from the interface between the solution layer and the substrate film, the thickness of the liquid crystalline polymer solution layer is continuously measured to monitor the state of application.