PURPOSE: To provide a coating device capable of on-line measurement of variations in the film thickness of coating liquid on the slide surface of a coater die during coating operation.

CONSTITUTION: The coating device 1 is provided with a back roll 3 for moving a web 2 therearound, a coater die 4 having a slide surface down which the coating liquid is allowed to flow and a coating liquid supply means 45 for supplying the coating liquid to the coating die 4. The coating liquid is allowed to flow down the slide surface of the coater die 4 for application to the moving web and variations in the film thickness of the coating liquid flowing down the slide surface of the coater die 4 are measured by a measuring means 20.