The surface potential of heat-treated polyimide (PI) Langmuir-Blodgett (LB) films deposited on various metal-electrodes was measured in a box shielded from light while nitrogen gas flowed through the box. The surface potential depended on base electrode material and the number of deposited layers. The surface potential saturated when the number of deposited PI LB layers was about 10. The saturated surface potential was a linear function of the work function of the metal. It was suggested that excess charges (electrons) were transferred from metal electrodes to heat-treated PI LB films until thermodynamic equilibrium was established at the metal-PI interface. The penetration depth of excess charges displaced from the metal electrodes into PI LB films was estimated to be about 4 nm. In order to clarify further the formation of a space charge layer in PI LB films at the metal-PI interface, the photoinduced surface potential in heat-treated PI was also measured. It was found that the surface potential changes as a result of the displacement of photogenerated carriers along the electric field produced by excess charges.