A plate-making sheet in the form of a strip having two longer-sides and two shorter-sides includes a base sheet and an adhesive sheet laminated on the base sheet. The adhesive sheet has a label portion formed in the middle thereof for being printed with an image. The label portion is defined by a cutting line formed in the adhesive sheet. A plate-making sheet group includes a plurality of types of plate-making sheets having respective label portions different from each other, which are distinguished from each other in respect of color of at least one of the base sheet and the adhesive sheet. A printing apparatus for printing a stamp image on the label portion of the plate-making sheet comprises a printing mechanism for printing a stamp image on the label portion, a feed mechanism for feeding the plate-making sheet through the printing mechanism, and a sensor for detecting feeding of the plate-making sheet into the printing mechanism. The length of a portion of a feed path of an ink ribbon, which extends from a point where the ink ribbon leaves the printing mechanism to a central point of a stamp surface of a stamp-making object material, is substantially equal to a half of a length of each of the two longer-sides of the plate-making sheet.