The electron transfer rate constant at electrode/liquid interfaces is theoretically described on the basis of the Anderson-Newns-Schmickler model. A compact formula for the rate constant is derived in the nonadiabatic limit, which is expressed in terms of the spectral density of surrounding media, the density of states of electrons in the electrode, and the weighted electronic coupling constant between the electrode and the redox couple in the liquid. The outer-sphere spectral density is then related to the experimentally accessible data on the frequency-dependent dielectric response functions of the solvent and the electrode with the aid of the dielectric continuum approximation. The derived formula provides a quantum-mechanical extension of the conventional nonadiabatic expression for the heterogeneous electron transfer reactions at electrode/liquid interfaces, taking into account the quantum effects associated with the high-frequency modes of both outer and inner spheres. On this basis, the quantum correction for the electron-transfer rate constant is numerically analyzed for some metal or semiconductor electrodes in contact with the Fe2+/3+ redox couple dissolved in water solvent at room temperature. In the case of zero energy gap, the quantum correction is found to be a factor of 4-5 for a typical configuration of the redox couple regardless of the species of electrode, while the rate constant itself is significantly affected by the dielectric property of the electrode. The energy gap dependence of the quantum correction is also discussed.