An image of a light beam, which has been totally reflected from an interface between a dielectric material block and a thin film layer of an analysis chip for supporting a sample, is detected with photodetector and as a two-dimensional image constituted of pixels arrayed in a beam width direction and an incidence angle direction, which are perpendicular to each other. An abnormal pixel row, which contains a pixel represented by abnormal pixel data, is extracted from among pixel rows, each of which extends in the incidence angle direction, in the array of the pixels constituting the two-dimensional image and in accordance with an output of the photodetector. A position of a dark line in the light beam, which has been totally reflected from the interface, is detected from the pixel data corresponding to the pixel rows other than the abnormal pixel row.