PURPOSE: To clearly mark a lot number or the like on a synthetic resin film bag easily and clearly without using any coloring agent, by a method wherein a carved seal is stamped on a predetermined part of a synthetic resin film bag to produce an image consisting of a rugged gradient shade.

CONSTITUTION: The bag made of a synthetic resin film (e.g., polypropylene) is placed on a soft backing plate having a minute rugged pattern made of a lead alloy, such as one provided for a checkwriter. The back side of the bag is moved upwards and, preferably, moved also in transverse directions, then it is brought into contact with a fixed metallic plate harder than the backing plate and provided with a carved seal for characters, symbols or the like, and the seal is stamped on the bag.