Transparent polypropylene (PP) decorative sheets make an ideal molding material in terms of recyclability. However, surface defects, particularly wrinkling, occur in PP sheets during the inmold injection molding process. In this study, we analyzed the occurrence of wrinkles in decorative sheets by visualization experiments of the molding process as well as clarified the relationship between wrinkle depth and molding conditions, and measured the sheet deformation amount and shear force between the sheet and cavity surface. We found that the wrinkle generation phenomenon of PP sheets was caused by sheet deformation along the flow direction with sheet slippage on the cavity surface after passage of the flow front. The sheet deformation decreased as if according to a power-law distribution in line with increasing screw injection speed and it occurred over the front contact area for only a very short time immediately after the flow front passed. Based on these results, we proposed a sheet deformation model, where at the moment the flow front passed, the perpendicular force F_Z and derived frictional force F_S to the cavity surface did not rise enough to fix the sheet onto the cavity surface. Therefore, the sheet slipped on the mold surface and deformed with thermal expansion and deformation stress caused by flowing melts. After the F_Z derived F_S rose enough to fix the sheet onto the cavity surface, the sheet deformation stopped. These deformations in micro regions were integrated and the sheet wrinkle which had occurred was thought to be due to the redundant length of the sheet.