Details of the three-dimensional morphology of the ichnofossil Phycosiphon incertum collected from deposits on submarine slopes are reconstructed by processing a series of images obtained from polished sections of the samples. Samples were collected from the mudstone around a slump scar in the Paleocene Shiomi Formation, northern Japan, which is characterized by the occurrence of slump scars. The reconstructed morphology of Phycosiphon incertum is a meandering tube with a flattened ellipse cross section. The tubes are flattened in a plane oblique to the bedding surfaces and aligned along the same direction at both the interior and exterior of the slump scar. Flattening of the tubes was likely caused by sediment compaction, and the tube flattens toward the horizontal plane that is oblique to the bedding plane because of the paleoslope inclination. The difference between the bedding and flattening planes of the tubes of Phycosiphon incertum may imply paleoslope inclination. When the inclination of the bedding plane of the Shiomi Formation is corrected using the flattened surfaces, the bedding plane dips by 9° toward the southeast, which conforms to the paleocurrent direction of the turbidites. The morphology of Phycosiphon incertum can, therefore, be used as a paleoslope indicator.