A method for producing an electret capacitor microphone high in sensitivity and stable in acoustic characteristic. A PET film stretched with predetermined tension is bonded to a diaphragm support ring to perform stretch and fixation of a diaphragm to produce a diaphragm sub-assembly. The diaphragm sub-assembly is heated at a predetermined temperature (e.g. 200.degree. C.) higher than a second order transition point of PET. Then, the diaphragm sub-assembly is packed in a housing. Because the diaphragm sub-assembly is heated at the predetermined temperature before packed in the housing, the stiffness of the diaphragm is reduced. Because the stiffness of the diaphragm 26 is reduced in such a manner, the tension at the time of stretch and fixation of the diaphragm can be set to a large value to prevent the diaphragm from being crinkled.