Linear ribs are formed radially with a center at a through-hole on one face of an X-ray transmissive film (radiolucent film) in an X-ray transmissive window (radiolucent window) to be used for an X-ray detector (radiation detector). The X-ray transmissive window faces a sample. A beam for irradiation to the sample passes through the through-hole, and X-rays (radiation) are radially emitted on a line extending through the through-hole and enter the X-ray transmissive window. Since the linear ribs are formed radially with the center at the through-hole, even X-rays entering at shallow angles with respect to the X-ray transmissive window are transmitted through the X-ray transmissive window at a probability equivalent to X-rays entering at deep angles. More X-rays are transmitted through the X-ray transmissive window, and thus the X-ray detector can detect X-rays with high efficiency.