The photosynthetic reaction center (RC) from the photosynthetic bacterium Rhodopsuedomonas viridis is a unique protein in which light-induced charge separation occurs. The charge-transfer process of the RCs was analyzed by a displacement current method in which two electrodes sandwiching the LB film without contact. A displacement current was detected for even a monolayer RC-LB film when it was irradiated with light. The action spectrum of the displacement current was in agreement with the absorption spectrum of solublized RCs, indicating that the current originates from the charge separation within the RCs. Furthermore, displacement currents for two types of LB films, in which the orientations of the RCs were opposite, yielded polarities that were opposite to each other. Thus, this method can be used to evaluate the unidirectional electron transfer in monolayers of RCs.