Diffusion of electron in polyimide LB films of Al / polyimide LB films / Au (MIM) structures is reported. Time dependences of the generated voltage in this MIM structures were proportional to the square root of time. Such a relation could be explained by the diffusion of electron from the Au electrode to the Al electrode through polyimide LB films. From the experimental results, the diffusion constant of electron was estimated as about 2.5x10(-17) (cm(2)/sec). Furthermore, the diffusion current calculated by the diffusion constant coincided with the current measured in the experiments. It is considered that the diffusion of electron is one of the main causes of the voltage generation in the MIM structures of polyimide LB films.