We have investigated the electric field dependence of the carrier photogeneration efficiency in an organic photoconductor with high sensitivity. The rate-determining step to generate photocarriers was considered to be the electron transfer between the neighboring two-redox molecules, which is influenced by the electric field. The overall carrier photogeneration efficiency was expressed by employing the electron-transfer velocity at the rate-determining step as a function of electric-field-dependent activation energy. This model successfully fits the experimental results for an organic photoconductor over a wide range of field strengths.