Abstract: A peak intensity of a (004) plane is greater than or equal to 3 times a peak intensity of a (110) plane in an X-ray diffraction pattern obtained by irradiation of X-rays to a membrane surface of an AFX membrane.

Abstract: A method for producing a transparent alumina sintered body according to the present invention includes (a) a step of preparing an alumina raw material powder containing a plate-like alumina powder having an aspect ratio of 3 or more and a fine alumina powder having an average particle diameter smaller than that of the plate-like alumina powder so that, when a mixing ratio of the plate-like alumina powder to the fine alumina powder in terms of mass ratio is assumed to be T:(100?T), T is 0.001 or more and less than 1, and so that a mass ratio R1 of F relative to Al in the alumina raw material powder is less than 15 ppm; (b) a step of forming a raw material for forming containing the alumina raw material powder into a compact; and (c) a step of sintering the compact so as to obtain a transparent alumina sintered body.