A sheet-decorating injection molding method using a sheet clamper for fixedly holding a decorative sheet on a parting surface of a movable mold of an injection mold. At least the inner edges of the surfaces facing the movable mold of the sheet clamper are finished in smooth, convexly curved surfaces, respectively, and a groove for receiving the sheet clamper therein is formed in a fixed mold at a position corresponding to the sheet clamper in a depth sufficient to permit the movement of the sheet clamper between a position to hold the decorative sheet fixedly on the parting surface of the movable mold and a position to be separated from the decorative sheet in state where the injection mold is clamped. The sheet clamper fixedly holding the decorative sheet is moved away from the parting surface of the movable mold and separated from the decorative sheet to release the decorative sheet at a timing during a period between completion of a mold clamping process and the start of a mold opening process and is moved further away from the parting surface of the movable mold as the movable mold is moved further away from the fixed mold after the start of a mold opening process.