To strengthen an outer package at a position where the largest stress is applied during transportation or the like by stretching a film by a first roller and a second roller which rotates at a larger peripheral rate than that of the first roller and also by varying a diameter of the second roller along a rotation axial line.

A stretchable film fed from a reel 3 is stretched by a deflection roller 5 (first roller) and an extension roller (second roller). The extension roller 7 is rotated at a peripheral rate along a tangent direction than that of the deflection roller 5. This rotation is driven by an electric motor 3 and decreasing gear strings 15.1 to 15.3. The extension roller 7 is a stepped roller, that is, its diameter varies along a rotation axial line. For example, it comprises a cylinder 19 and a conic part 21, wherein the cylinder 19 normally extends into a region 17.1 where the film 17 is stretched in the lengthwise direction, while the conic part 21 forms a region 17.2 with a small extension. The respective regions 17.1, 17.2 are made to correspond to a stress position applied to a parcel.