A tensioner having a simplified structure, using engaging teeth having increased strength and reduced backlash, constructed from a reduced number of parts, reduced in cost, and having increased design freedom. A tensioner comprises: a tubular member (1) having engaging teeth (1b) formed thereon; one or more engaging elements (2) having formed thereon engaging teeth (2a) engaging with the engaging teeth (1b); and a shaft member (3) provided inside the tubular member (1) and having an engaging-element receiving section (3c) or an engaging-element supporting section which receives the engaging elements (2) of the tubular member (1). Either the tubular member (1) or the shaft member (3) functions as an advancing member which is moved by urging force in an advancing and retracting manner. The tensioner is provided with a ratchet mechanism which, when the engaging elements (2) move in a diameter reducing direction and move over the engaging teeth of the tubular member (1), allows the advancing member to advance forward, and when the engaging elements (2) move in a diameter expanding direction and engage with the engaging teeth of the tubular member (1), prevents the advancing member to retract.