An apparatus for anodizing substrates immersed in an electrolyte solution. A substrate holder mounted in a storage tank includes a first support unit having first support elements for supporting, in a liquid-tight condition, only lower circumferential portions of the substrates, and a second support unit attachable to and detachable from the first support unit and having second support elements for supporting, in a liquid-tight condition, remaining circumferential portions of the substrates. A drive mechanism separates the first support unit and the second support unit when loading and unloading the substrates, and for connecting the first support unit and the second support unit after the substrates are placed in the substrate holder.