A measurement path is filled with air prior to performing actual measurement. A p-polarized light beam is caused to enter an interface, and the intensity distribution of the light beam reflected at the interface is detected by a photodiode array to obtain a reference intensity distribution of the light beam itself. Thereafter, the measurement path is filled with a target for measurement, and the intensity distribution of a light beam reflected at the interface is measured. Each of the measured distribution values are divided by the reference intensity distribution, to cancel out influences due to fluctuations in the intensity distribution of the light beam. Thereby, the position of an attenuated total reflection angle is detected with high accuracy. Because a light beam constituted by p-polarized light waves is utilized, separating means for separating the light beam reflected at the interface into p-polarized and s-polarized light waves becomes unnecessary.