To surely detect a connection part by a phototube detection device by forming an uneven pattern on one side or both sides of a heat-sealed connection part to implement the irregular reflection of the light, and increasing the difference in transmission of the light between the connection part and a film part other than the connection part.

In a device to recoil a raw film folded in a translucent plastic cylinder from a roll, and continuously feed it to a bag-making machine, an uneven pattern to reduce the transmission of the light is formed on one side or both sides of its connection part S when one volume of the previous raw film W1 is completed, and a starting end of a next raw film W2 is connected to a rear end of the previous raw film through the heat seal. The connection part can surely be detected by a phototube detection device comprising a light- emitting part 6 and a light-receiving part 7 provided on a passage of the film of the bag-making machine, an extra labor of attaching a tape can be saved, and defective bags are prevented from being fed to a filling device.