A sheet post-processor of this disclosure is provided with a punching device and a registration roller. The punching device is capable of forming punch holes along a first side end edge of a sheet parallel to a sheet width direction orthogonal to a sheet conveyance direction and a second side end edge of the sheet parallel to the sheet conveyance direction. The registration roller is disposed downstream of the punching device in the sheet conveyance direction. The punching device includes a punch unit that forms punch holes through a sheet and an edge sensor that detects the second side end edge of a sheet. In a case of forming punch holes along the second side end edge of a sheet, without skewing of the sheet being corrected by the registration roller, the edge sensor detects the second side end edge, while the punch unit forms punch holes through the sheet.