An electrolytic bath includes a shell, an upper cover joined on an upper end of the shell, an outer supporting member held in the shell, a hollow cylindrical positive electrode plate held in the outer supporting member, a hollow cylindrical negative electrode plate held in the positive electrode plate, an inner supporting member held in the negative electrode plate, a lower cover joined on a lower end of the shell, and a base member on a bottom of the lower cover; a space exists between the positive and the negative electrode plates; the electrolytic bath has a waste water outlet and a water outlet hole for acid waste water and alkaline water to flow out therethrough respectively; because the electrode plates are hollow cylindrical and completely open, they can't change shape easily, and there is no need for a separating plate, and scale can't form on the electrode plates easily.