PURPOSE: To obtain a transmission type diffuser which can transmit and diffuse even a light having short wavelength by forming microparticles having a specific particle size on a thin film transmitting soft X-rays.

CONSTITUTION: Light beam from a high output pulse laser 1 is condensed through a condenser lens 2 and projected onto a target 5 placed in a high vacuum container 4 through an incident window 3 thus generating soft X-rays 6. The soft X-rays pass through a rotary elliptical oblique incident condenser mirror 7 and impinge on a diffuser 8 where the X-rays are diffused. The X-rays then pass through a thin film window 10 and impinge on a sample 12. Subsequently, the X-rays transmit through a thin film window 11 and impinge on a Schwarzschild objective lens 13 to be focused on an image detector 14. The diffuser 14 is produced by forming microparticles having particle size 1-1000 times as large as the wavelength of soft X-rays on a thin film transmitting the soft X-rays. This constitution allows diffusion of light having short wavelength, which is absorbed by a conventional transmission type diffuser, in Mie scattering region where the loss of the quantity of light is low.