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We find that a non-axisymmetric bow shock model, with an appropriate choiceof parameters, could fit the line splitting velocity field of the cone-likeH$\alpha$ nebula in NGC 4945 better than a canonical cone model. Meanwhile, thebow shock model could also reproduce the morphology of the H$\alpha$ nebula.The bow shock results from the interaction of the galactic superwinds with agiant HII region. It is implied that the starburst ring or disk around thegalactic nucleus should be generating strong winds, and the bright H$\alpha$knot northwest of the nucleus be suffering an anisotropic mass loss process.Comment: 14 pages, aasms4.sty, 3 figures not included (available upon request)  To appear in ApJ Letters. email chy, jhh@crater.nju.edu.c

The Conelike Hα Nebula in NGC 4945: A Galactic Superwind Bow Shock?