The inventive method provides a membrane for X-ray lithography compositely composed of silicon carbide and silicon nitride having high performance in respect of stability against high energy beam irradiation and transparency to visible light. The method comprises depositing a composite film of a specified SiC:Si.sub.3 N.sub.4 molar proportion by sputtering on a silicon wafer as the substrate under such conditions that the thus deposited film is under a compressively stressed state, annealing the substrate bearing the composite film deposited thereon at a specified temperature so as to bring the composite film under a tensile internal stress and then removing the substrate by etching leaving a frame portion.