High precision cosmic-ray antiproton fluxes recently reported by BESS-Polar and PAMELA are consistent with secondary production from interactions of primary cosmic rays with the interstellar medium. This severely constrains the possibility of antiprotons of primary origin such as annihilation of supersymmetric dark matter or evaporation of primordial black holes. In the case of antideuterons, however, secondary production in collisions is strongly suppressed, especially at low energies, because of the very low production cross-section and strict kinematic requirements compared to antiproton production. The lack of secondary background would imply that there is still plenty of room to search for primary antideuterons from novel production processes. In spite of such attractive features, antideuterons detection is enormously challenging since the expected flux is extremely small and superior particle identification is necessary to separate antideuteron candidates from the antiproton background. The most sensitive reported search used BESS flight data obtained during the previous solar minimum period in 1997. By comparison, the BESS-Polar II flight in 2007/2008 accumulated cosmic-ray data in near solar minimum conditions with more than ten times the statistics of BESS97. We will report the result of a new search for antideuterons with unprecedented sensitivity using BESS-Polar II.