The ferroelectric liquid crystal FLC-117, which is known to exhibit at a certain temperature the sign inversion of spontaneous polarization P-s, is studied under an electric field. From the analysis of the electro-optic response and the direct texture observations, it is concluded that the inversion temperature of P-s has the applied voltage dependence. To interpret our experimental result, we introduce a coupling term between the molecular dipole moments and the magnitude of the applied electric fields to the asymmetric rotational potential about the molecular long axis. The simulated results using this potential are in accordance with our experimental result.