Abstract: A sample is placed on a sample receiving stage for receiving the sample. The sample is transferred one at a time to a measuring table, and a thickness of a thin film formed on a surface of the sample is measured by irradiating the surface of the sample with a measuring light beam. A covering structure is disposed between the sample receiving stage and the measuring table to define a sample transfer space and a measuring space in which the measuring table is disposed. A high-purity purging gas containing only a very small amount of contaminants is supplied through purging gas supply devices into the transfer space and the measuring space covered with the covering structure.