To provide a method which can manufacture a soap simply without using dangerous sodium hydroxide.

This method is characterized in that the alkaline water A, which is generated by the electrolysis of water containing sodium salt as an electrolyte, is heated with the oil and fat Y to manufacture the soap S. An electrolyte such as sodium chloride and sodium bicarbonate added in water is electrolyzed to produce the alkaline water A at the site of the cathode 22, which can be heated with the added oil and fat Y to manufacture the soap because the water A contains sodium ion and the other ion such as hydroxide ion.