The dead time of a pulse type X-ray detector is measured without estimation of a true X-ray intensity. The first and the second conditions are used for varying an intensity of an X-ray entering the X-ray detector. The first condition may be the slit width of a receiving slit, at least three kinds of slit width being selected. The second condition may be with or without an absorption plate. The first observed X-ray intensities are observed, with the absorption plate inserted, for three or more values in slit width. Next, the second observed X-ray intensities are observed similarly but with the absorption plate removed. A predetermined relational expression is made up among the first observed X-ray intensity, the second observed X-ray intensity, a ratio k of the second observed X-ray intensity to the first observed X-ray intensity (depending upon attenuation in X-ray intensity caused by the absorption plate) and the dead time .tau. of the X-ray detector. Based on the relational expression, a fitting operation is carried out with the least squares method so as to determine the dead time .tau. precisely.