A film thickness measuring device is provided with a light source, a spectroscopic sensor, a processor, and a storage unit, and configured in such a manner that light from the light source vertically enters a plane to be measured provided with a film and the light reflected by the plane to be measured enters the spectroscopic sensor. The storage unit stores theoretical values of reflectivity distributions of respective film thicknesses and theoretical values of color characteristic variables of the respective film thicknesses. The processor finds the thickness of the film of the plane to be measured from the reflectivity distribution measured by the spectroscopic sensor by using the theoretical values of the reflectivity distributions of the respective film thicknesses or the theoretical values of the color characteristic variables of the respective film thicknesses stored in the storage unit.