Alkali metal hydroxide is produced by electrolysis of alkali metal chloride according to an improved diaphragm process wherein plural cationic ion-exchange membranes are used to divide an electrolytic cell into an anode compartment, at least one middle compartment and a cathode compartment. The ion-exchange membrane confronting anode is preferably substantially resistant to anode reaction products such as chlorine or chlorates. Alkali formed by electrolysis is recovered from cathode compartment or alternatively a portion thereof may be taken out from middle compartments at various concentrations. Alkali metal hydroxide can be produced in high purity and high concentration with excellent efficiency.