To provide an in-mold decorating apparatus for injection molding having a sheet cutter using a heating wire generating no slackening.

In an in-mold decorating apparatus for injection molding equipped with a sheet cutter having a heating wire 63 cutting the decorating sheet continuing to a decorating sheet supply part from the parting surface of a female mold at the position on the side of the decorating sheet supply part of the female mold when a hot platen 60 movable to the position along the parting surface of the female mold is present at the position pressed to the parting surface of the female mold, the sheet cuter is provided under tension between the support members 61 provided to the hot platen 60 in such a state that tension is applied to the heating wire 63 by the spring 63a provided to at least one of the end parts of the heating wires 63. Since the generation of slackening in the heating wire 63 is prevented by the tension of the spring 63a, it can be prevented that the heating wire 63 comes into contact with the mold to generate a short circuit and the decorating sheet can be certainly cut linearly.