the change in the sample size and a change in background intensity due to the coexisting element are measured in real time to thereby automatically change a measurement time , the detection lower limit is kept constant , so that a fluorescent x - ray apparatus is provided that is capable of measuring every time in the same detection lower limit even in a case where there have existed a change in size of a sample , a change in sensitivity due to a difference in main ingredient , and a change of a magnitude in background due to an influence of a coexisting element .