The Langevin equation for the superconducting order parameter above the transition temperature ${T}_{c}$ proposed by Schmid is modified to calculate the nonlinear excess conductivity ${\ensuremath{\sigma}}^{\ensuremath{'}}(T,E)$ slightly below ${T}_{c}$. The electric field dependence of ${\ensuremath{\sigma}}^{\ensuremath{'}}(T,E)$ is described approximately by the same function of $\frac{E}{{E}_{c}}(T)$ as Schmid's function above ${T}_{c}$ with a newly defined characteristic field ${E}_{c}(T)$ below ${T}_{c}$. The experimental results of the nonlinear electrical conductivity of aluminum films below ${T}_{c}$ are in fairly good agreement with the theory.