Application of an intense light field to solids produces enormous and ultrafast nonlinear phenomena such as high-harmonic generation1,2 and attosecond charge dynamics3,4. These are distinct from conventional photonics. However, the main targets for investigation have been limited to insulators and semiconductors, although theoretical approaches have also been developed for correlated metals and superconductors5. Here, in a layered organic superconductor, a nonlinear charge oscillation driven by a nearly single-cycle strong electric field of >10 MV cm−1 is observed as a stimulated emission. The charge oscillation is different from a linear response and ascribed to a polar charge oscillation with a period of ∼6 fs. This nonlinear polar charge oscillation is enhanced by critical fluctuations near a superconducting transition temperature and a critical end-point of first-order Mott transitions. Its observation on an ultrafast timescale of ∼10 fs clarifies that Coulomb repulsion plays an essential role in the superconductivity of organic superconductors.