PURPOSE:To stably elecrolyze salt water over a long period at low voltage with high current efficiency by keeping the concn. of Ca in the salt water supplied at below a specific value when the electrolysis is carried out using a fluorine- contg. cation exchange resin membrane with a predetermined ion exchange capacity while keeping NaOH at a specific concn. CONSTITUTION:Salt water is brough into contact with a chelate resin layer such as EDTA to lower the concn. of Ca in the water to below 0.08 mg/l and that of impurities such as Mg to below 0.04 mg/l. The purified salt water is then electrolyzed using a fluorine-contg. cation exchange resin membrane having carboxylic acid groups as exchange groups and an exchange capacity of 0.8-2.0 milliequivalents/ gram of dry resin while keeping the concn. of NaOH in the cathode chamber at 20- 45wt%. Thus, adhesion or accumulation of Ca to the membrane is controlled, and a rise in electric eresistance of the membrane, a lowering of current efficiency and deterioration in mechanical strength are prevented, whereby NaOH of high concn. can be produced.