To provide a new thin substrate for a cataplasm, alternative to a conventional cloth or nonwoven fabric, and preventing the decrease in a medicinal ingredient stemming from its migration thereinto and preventing the efficacy sustainability of the above ingredient stemming from its dispersion into the air, reducing skin irritancy in its application, and also followable to skin mobility after its application, and to provide a cataplasm using the above substrate.

The new substrate for cataplasm includes a base film layer 5-300 m in thickness stretchable at room temperature and a polyester resin film layer 1-25 m in thickness disposed on the base film layer, wherein the polyester resin film layer has grooves variable in width in response to the stretching of the base film layer. The cataplasm using this substrate is also provided.