Molecular orientation of photosynthetic reaction center protein (RC) was controlled during film deposition in Langmuir-Blodgett (LB) film preparation by the polarity of an applied bias voltage. The orientation of RCs was examined by measuring a displacement current. The effects of counterions in the subphase (including both bivalent cations and polyions) were studied to clarify the mechanism involved in orientation control by an applied bias voltage. Results confirmed that the orientation of an RC at the water surface was governed by the electrostatic repulsion between the electric double layer near the substrate and the dipole of the RC.