PURPOSE: To increase X-ray intensity for exposure, by applying quasi- monochromatic X-ray composed of wavelength component in a narrow band whose center wavelength gives a high reflection factor of a reflecting mirror system, to an incident X-ray.

CONSTITUTION: Reflected X-ray from a mask 7 reflecting quasi-monochromatic X-ray outputted from an undulator 5 is demagnified by a reflecting mirror 8, and projected on a wafer 9, thereby forming a pattern of the mask 7 on the wafer. By the undulator 5 inserted in an electron storing ring 1, the stable quasi-monochromatic X-ray of high luminance is generated. Undulator magnetic field or electron beam energy is so adjusted that the center wavelength becomes equal to the wavelength giving a high reflection factor of the reflecting mirror 8, or further equal to the wavelength giving the maximum reflection factor. Thereby the energy absorbed by the reflection mirror system is reduced, so that requirements for heat resistance and radiation resistance of the mirror can be reduced. Since a reflection mask is used as the mask 7, the strength of a mask can be increased.