Observation samples in a sample solution are held due to absorption or the like on a rear face of a first X-ray transmission film. In a mirror body, while an X-ray emission film and X-ray transmission films are bent to be convex outward due to a pressure difference, an X-ray transmission film is bent to be convex toward the X-ray transmission film side due to gas expansion in a second cavity part. This bending results in widening of a gap between the first and second X-ray transmission films in their center part more compared with a gap between their end parts. However, there is almost no change between the X-ray transmission films.