Abstract: A method for measuring a film thickness of a sheet including feeding a sheet over a rotary shaft, providing a light shield parallel to the center of rotation of the rotary shaft at a predetermined distance from the rotary shaft surface to define a gap therebetween, scanning the gap with a laser beam, and measuring the time during which the laser beam passes through the gap to determine the film thickness of the sheet to be measured. A different-diameter portion is provided at the end portion of the rotary shaft for calculating a difference between the time during which the laser beam passes through the gap on the different-diameter portion and the time during which the laser beam passes through the gap on the portion of the rotary shaft adjacent to the different-diameter portion and on which no sheet is placed, and correcting the time during which the laser beam passes between the surface of the sheet and the light shield by utilizing the calculated difference to obtain the thickness of the sheet.

Abstract: A film thickness measuring method and device therefor which are free from drawbacks caused by errors in a sheet conveying system and an optical system of the device and the like. The optical system is moved over a sheet having a sheet member and a film formed thereon, whose thickness is to be measured, so that the deformation of the conveying system and the optical system is detected in advance. After taking the detection results into an account, the thickness measurement is carried out resulting in improving an accuracy thereof.

Abstract: A film thickness measuring device for measuring the thickness of a film formed on a sheet member conveyed with the rotation of a rotary shaft. The thickness measurement is accomplished based on averaged values for a period of time which is determined according to the rotation speed of the rotary shaft. Consequently, errors due to the eccentricity or uneven surface of the rotary shaft are removed from the measurement values resulting in improving an accuracy of the thickness measurement of the film.