A web separating device includes a cutting/joining drum, which is rotatable in contact with web of optical film on a peripheral surface thereof. A cutting drum is positioned for contacting the cutting/joining drum peripherally, and has a cutter for cutting the web being supplied in a width direction in cooperation with the cutting/joining drum, to obtain upstream and downstream web sections. A joining drum is positioned for contacting the cutting/joining drum peripherally, and attaches a guide leader to a front end of the upstream web section. Guide rollers are disposed for allowing passage of the web and the upstream and downstream web sections in a non-contact manner before and after being cut or attached.