PURPOSE: To accurately control accumulated illuminance of X-ray irradiated onto a wafer through a mask by providing a calculating means to calculate the relationship between an X-ray illuminance generated from an X-ray source and an X-ray illuminance irradiated onto a wafer through a mask, and a controlling means to control X-ray illumination irradiated onto the wafer based on the calculation results.

CONSTITUTION: A calculation means to calculate the relationship between an X-ray illuminance generated from an X-ray source 1 and an X-ray illuminance irradiated onto a wafer 6 through a mask 5, and a controlling means to control an X-ray illuminance irradiated onto the wafer 6 based on the calculation results by the calculation means are provided. For instance, output of an X-ray detecting sensor 4a provided at a side of the X-ray source 1 and an X-ray detecting sensor 4b provided at an area around the wafer 6 is took in an illuminance controlling circuit 9 and a ratio K of the X-ray illuminance by one X-ray detecting sensor 4a and the X-ray illuminance by the other X-ray detecting sensor 4b is calculated in advance. At the time of exposure, X-ray generation is interrupted when the accumulated illuminance on the wafer 6 attains a specified value based on the X-ray illuminance measured by the X-ray detecting sensor 4a and the ratio K.