We report experimental observations of a novel kind of convective oscillation in binary and ternary polymer solutions confined in a very thin layer (0.1 mm). This convective oscillation was considered to be driven by the Soret effect, which has been recently predicted by theory for a fluid mixture with a very small value of the Lewis number, namely, having much smaller mass diffusivity as compared to thermal diffusivity. For binary polymer solutions of polystyrene (PS) in dioctyl phthalate (DOP) and for ternary solutions of PS/PB/DOP (polybutadiene (PB)), the magnitude of convective flow was oscillating with time until reaching a stationary state.