PURPOSE: To make fluorescence excitation light of soft X-rays, provide a dichroic mirror for transmitting the soft X-rays and reflecting the fluorescence light excited with said soft X-rays and a soft X-rays fluorescence microscope equipping it and make possible the observation of a thick specimen which cannot be observed with a conventional soft X-rays microscope.

CONSTITUTION: Soft X-rays are made fluorescence excitation light and a thin film of a dichroic mirror is constituted in order that the soft X-rays may transmit it and fluorescence excited with said X-rays may be reflected thereon. A soft X-ray fluorescence microscope equipping the dichroic mirror has a fluorescence excitation light source 1, the dichroic mirror 2 formed of a thin film, an objective lens 3 and a specimen 4 on the same light axis 9. The others include a light (image) detector 5 and a barrier filter 6.