We propose a method that can determine the ellipticities of source stars of microlensing events produced by binary lenses. The method is based on the fact that the products of the caustic-crossing timescale, Δt, and the cosine of the caustic incidence angle of the source trajectory, κ, of the individual caustic crossings are different for events involving an elliptical source, while the products are the same for events associated with a circular source. The product Δt_⊥ = Δt cos κ corresponds to the caustic-crossing timescale when the incidence angle of the source trajectory is κ = 0. For the unique determination of the source ellipticity, resolutions of at least three caustic crossings are required. Although this requirement is difficult to achieve under the current observational setup based on alert/follow-up mode, it will be possible with the advent of future lensing experiments that will survey wide fields continuously at high cadence. For typical Galactic bulge events, the difference in Δt_⊥ between caustic crossings is of the order of minutes, depending on the source orientations and ellipticities. Considering the monitoring frequency of the future lensing surveys of ～6 times hr~(-1) and the improved photometry, especially of the proposed space-based survey, we predict that ellipticity determinations by the proposed method will be possible for a significant fraction of multiple caustic-crossing binary lens events involving source stars having nonnegligible ellipticities.
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