Abstract: A chain saw guide bar has a pair of elongated chain guide plates, which are superimposed with each other and receive a saw chain. Positioning slots extend in the longitudinal direction of the guide plates and receive a pair of fixing stud bolts that project from the main body of the chain saw. A chain-tightening device, which is incorporated into the guide bar, includes a pulley carried by the guide plates in a fixed position distally of the guide slots. One end of a flexible wire is connected to the pulley. The other end of the flexible wire is connected to a circular ring that is slidably received between the guide plates with an opening thereof at least partly registering with the positioning slots. The ring is received on one of the fixing stud bolts. An intermediate portion of the wire is turned partway around a returning member that is stationarily located on a portion of the guide bar proximally of the fixing stud bolt that receives the ring.