In the electrode position on the vertical plate cathode immersed in an unstirred electrolyte, a concentration boundary film is formed along the cathode surface and the cathodic limiting current density is determined by the transfer rate of copper ion through the film. Moreover, the decrease of copper ion concentration in this film causes an upward natural convective flow of electrolyte along the surface and the average cathodic limiting current density is expressed by equation (6). The effects of copper ion concentration in the bulk electrolyte and of the height of the cathode on the cathodic limiting current density can be interpreted by this equation. The natural convective flow becomes turbulent when the Reynolds number concerning the film thickness exceeds 200 and the experimental results were summarized by an equation composed of dimensionless numbers of Sh, Sc and Gr. The apparent activation energy of diffusion of copper ion in the electrolyte was estimated at 4.8 ㎉/㏖.