To provide a radiation spectrum measuring system capable of reducing the influence of a tail distribution in a pulse height distribution formed by an excitation X-ray, and lowering the measurement lower limit of an element to be measured.

A rise time detection module 26 compares a rise time of a pulse signal output from a proportional counter 11 with a preset value set previously. Processing is performed so that a pulse signal having a longer rise time of the pulse signal than the preset value is not taken as data by a pulse height analysis module 23. Consequently, the influence of the tail distribution in the pulse height distribution formed by the excitation X-ray can be reduced, and the measurement lower limit of the element to be measured can be lowered.