A window member (11) having an intentionally designed distribution of refractive index is used, as a transparent window member in a light transmission area of a polishing pad for detecting the end point of polishing by a CMP method. This window member (11) has areas (11a) having a high refractive index and areas (11b) having a low refractive index in its window face. In a cross section normal to the window face, the high-refractive index areas (11a) and the low-refractive index areas (11b) are alternately arranged in stripes. These areas (11a and 11b) in the window face are in a Fresnel zone plate arrangement in which the first area (center circle) is a bright one (area having a high refractive index). A plurality of such Fresnel zone plates (F) are arrayed in a matrix in the window face of the window member (11).