In a measuring method utilizing the phenomenon of attenuation in total internal reflection in which a light beam is caused to enter a dielectric block provided with a film layer to be brought into contact with a sample so that total internal reflection conditions are satisfied at the interface of the dielectric block and the film layer and various angles of incidence of the light beam to the interface of the dielectric block and the film layer can be obtained, and the intensity of the light beam reflected in total internal reflection at the interface is detected, the light beam is caused to intermittently impinge upon the dielectric block.