The in-plane dispersion of the hole subband (HSB) in a Si quantum well is obtained for the first time by applying angle-resolved photoemission spectroscopy and surface science techniques. The entire shape of the HSB over a wide ranged wave vector, including admixing of heavy and light hole subbands around the crossing point and the camelback structure inducing negative effective mass, is visualized directly. Energy separations between the subbands are quantitatively explained.