It is generally hard to determine the tungsten content by the fluorescent X-ray analysis with a tungsten target x-ray tube, because of the interference of scattered radiation of the primary x-ray., However, the scattered radiation can easily be eliminated by applying a filter before the window of the x-ray tube.Any ordinary x-ray fluorescent instrument has no attachment for fixing of the filter in operation.Therefore the filter is practically of no use.A primary beam filter which can be operated at the panel during the operating time of the x-ray tube was developed, and the determination of tungsten in high chromium-tungsten steel by a tungsten tube was successfully carried out with it.As a result, the daily analysis became facile and the accuriacy of measurements was improved.