A method is proposed for designing an x‐ray spectrometer which can scan a limited wavelength region without movement of the concave grating or the electronic counter. Spectrometers widely used in the past have consisted of a fixed x‐ray source and a movable counter. This method is to use a fixed counter and a scanning x‐ray source to avoid undesirable movement of the sensitive electronic counter with its delicate and complex cooling system. The scanning x‐ray source will be obtained by scanning the cathode ray beam of an x‐ray tube. Two cases are analyzed theoretically; the Rowland circle (or Paschen‐Runge) mounting, and a new arrangement which will be designated as the Sawada mounting. In the latter, the concave grating and the middle point of the scanning region of the x‐ray source lie on the circumference of a circle with the same radius as that of the grating. It will be useful for constructing an x‐ray grating spectrometer in the future. The length of a spectral line is shorter in the case of the Sawada mounting than in the case of the Rowland circle mounting, and the width of the line is broader in the case of the Sawada mounting than in the case of the Rowland circle mounting, owing to various abberations. The other advantages and disadvantages of the Sawada mounting are discussed in comparison with the Rowland circle mounting.