To provide a radiation source fluctuation correction method capable of measuring at high accuracy by performing operation after removing data selecting the output value from a detector for instant-fluctuation (or spike) of radiation ray.

In the radiation source fluctuation correction method of a radiation generator, a radiation detector detecting radiation output from the radiation generator concerned, and the radiation detector grouping sequentially the data continuously output from the radiation detector concerned at every predetermined time to implement additional averaging for outputting a plurality of data contained in the grouped domain concerned, intensity of radiation is calculated using additional averaging value after removing the data selected in the grouped domain.