To provide a film thickness measuring device improving the accuracy of on-line measurement, and also to provide a film thickness measuring method.

A film thickness measuring device 100 measures the thickness of a web 20 on-line. For measurement of a film thickness, the film thickness is measured in advance during low speed transportation with a small influence of entraining of air, and the measured value is regarded as a master value (S02). After the transportation speed reaches a production target transportation speed, the difference between a measured value of the film thickness by on-line measurement and the master value, that is, the change amount is determined for measurement during high speed transportation (S05). After calculation of the change amount, the measurement result of the film thickness obtained during the high speed transportation is corrected with the change amount (S06).

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