Time dependence of density fluctuation for neat supercritical ethylene (C2H4) is investigated by dynamic light scattering at various densities in isothermal conditions of reduced temperatures T-r=T/T-c=1.02, 1.04, and 1.06. Time correlations of the density fluctuation for all thermodynamic states decay as single exponential functions with the time constant of submicrosecond. Critical slowing down of diffusive motions is observed in the time domain, and the correlation time of the density fluctuation becomes maximum at the extension of the gas-liquid coexistence curve on the P-T phase diagram. It is revealed that the time dependence of the density fluctuation just corresponds to the magnitude of static density fluctuation obtained by small-angle x-ray scattering measurements. By measuring correlation times as a function of scattering angle, the critical slowing down is thermodynamically discussed. It is elucidated experimentally that the critical slowing down of diffusive motions considerably correlates to the increase of specific heat capacity, the decrease of the thermal diffusivity, and rotational relaxation time in the supercritical state of the neat fluid.