A foil-decorating injection molding method and a foil-decorating injection molding apparatus adjust the tautness of a decorative sheet fed onto a parting surface of a female mold so that tensions T1, T2 and T3 of the decorative sheet during a sheet feed operation of the sheet feed means, at the start of the sheet clamping operation of the clamper for pressing and fixing the decorative sheet and at a moment immediately before the completion of the sheet clamping operation, respectively, meet an inequality: T1≧T2>T3 to prevent the decorative sheet from breaking or slipping off the clamper when clamping the decorative sheet on the parting surface of the female mold.