A skew roller is disposed near an abutment reference member. A pre-skew roller is disposed on the upstream side of the skew roller at a position corresponding to a conveyance center line. When a sheet conveyed by a pre-registration conveyance roller reaches the skew roller, the pre-registration conveyance roller moves upward to release a nipping force applied on the sheet. At the same time, the pre-skew roller moves downward to start aligning the sheet.