Electrochemical impedance spectroscopy has been applied to study atmospheric corrosion. A two electrode cell type atmospheric corrosion probe has been constructed. Quantitative analysis on IR drop and current distribution on the electrode, which produce serious errors in electrochemical measurements of corrosion rate of metal with a very thin moisture layer, have been made on the basis of transmission line model. The analysis reveals the optimum cell sizes and the limitation of atmospheric corrosion monitoring. The instantaneous corrosion rates of copper in the atmosphere of 30% to 95% Relative Humidity (RH) have been monitored by the continuous measurements of the impedances at 10mHz and 10kHz. Below approximately 60%RH, the corrosion rate shows extremely low values. When the atmosphere is switched from 60%RH to 70%RH, the corrosion rate abruptly increases, and then reaches a maximum value at 90%RH. The further increase of relative humidity slightly decreases the corrosion rate. These phenomena can be explained by the dependence of atmospheric corrosion rate on the thickness of electrolyte layer.