Pitting corrosion with fractal porous structure often occurs on copper tubes used in heat exchangers of air-conditioning equipment, and this corrosion is called an ant-nest corrosion. Galvanostatic controlled EIS (G-EIS) was used to measure 3-dimensianal (3D) impedance during the formation of ant-nest corrosion on phosphorus deoxidized copper (pd-Cu). Instantaneous impedance at an arbitrary time after galvanostatic polarization was determined from the 3D impedance. The transmission line model (TLM) for fractal porous electrode was applied to detect a branched pore formation on pd-Cu electrode. Nyquist plot of instantaneous impedance at initial stage of ant-nest corrosion shows capacitive behavior and straight line of 45° angle against real axis in high frequency region, indicating that a porous structure was formed at the initial stage. The angle of straight line was varied from 45° to 22.5 and 11.25°, indicating the formation of branched and doble-branched porous structures, respectively.