Flow-induced orientational changes in a 0.5 wt % Xanthan gum solution in planar channels with an abrupt expansion were examined by measurement of the flow-induced birefringence and velocity fields. Three kinds of 1:4 abrupt expansion channels with different cross-sectional aspect ratios (of 1, 2, and 5) in the upstream region were tested in the experiments. A similar channel with a different size but the same aspect ratio of 1 was tested for comparison. Remarkable differences were found in the development of flow-induced orientation near the centerline after the abrupt expansion, depending on the aspect ratio. In the cases of the aspect ratios of I and 2, the polymer molecules were temporally aligned perpendicular to the flow direction due to negative elongational flows generated after the abrupt expansion. In contrast, similar phenomena were not observed with an aspect ratio of 5. From these results, the typical flow-induced orientational changes in polymers in the planar channels just after the abrupt expansion were found to be substantially affected by the cross-sectional aspect ratio in the upstream region.