An extremely thick and porous oxide film can be obtained on Type304 stainless steel. The material was polarised in 5 M H2SO4 solution at 50-80 degrees C with applied potential modulated as a square wave. The porous oxide and/or hydroxide films were formed when the lower potential of the square wave, E-L, is in the active region of Fe and Cr, and the higher potential, E-H, in the passive region of Cr; this is noted as an anodic type, or when E-H is in the transpassive region of Cr and E-L in the passive region of Cr, that is, the cathodic type. The electrochemical impedance and photo-electrochemical response were examined for both films. For the anodic type film, the capacitance is larger than that of the substrate steel, and is nearly proportional to the total thickness of the film. In addition, the larger capacitance was observed for the film with the finer porous structure. Therefore, the film \ electrolyte interface, at which the electrochemical capacitance arises, is distributed across the whole cross section of the porous film. On the other hand, the capacitance of the electrode with the cathodic type film was almost equivalent to that of the substrate steel. Therefore, the capacitance arises only at the substrate \ solution interface. The photo current of both anodic and cathodic porous films exhibited p-type semiconductor responses. The photocurrent for the anodic porous him increased with increasing film thickness, then passed through a maximum at a thickness of about 10 nm, and finally decreased. On the other hand, the photocurrent for the cathodic film remains approximately constant with increasing thickness. The electrochemical impedance and the photoelectrochemical response indicated that the thick porous film has various electrochemical properties depending on the structure, which can be ordered under electrolysis conditions.